
W* «. CSJMHTON 


MS. W. CRAIG 


^OBIiSHED BY 

s«c!nti©n TeclMikal Institute 

¥. M. C. A. Maas. 









A. GENERAL TREATISE 


ON 


ELECTRICITY 

AND 

ELECTRICAL APPARATUS 


BY 

W. B. CLAYTON and JAS. W. CRAIG 

" I 

Associate Members American Institute of 
Electrical Engineers 

BOSTON, MASS. 


FIRST EDITION ... SECOND THOUSAND 



PUBLISHED BY 

ASSOCIATION TECHNICAL INSTITUTE 

YOUNG MEN’S CHRISTIAN ASSOCIATION 


LYNN, MASS. 

E. Newton Smith, Director 




In preparing the text, the Authors have had 
valuable assistance from the following Engineers: 

J, P. Catlin, E.E. 

A. DeForest Davis 
J. M. Brodie, B. S. 

H. P,. Hastings, E. E. 



i 


All rights reserved 


Copyright, 1910, by 

YOUNG MEN’S CHRISTIAN ASSOCIATION 
Lynn, Mass. 



I 

C Cl A 2 7 8 8 8 •! 


PRESS OF THOS. P. NICHOLS & SONS 
LYNN, MASS. 



PREFACE 


The Authors wish to avail themselves of this opportunity 
to give a few words in explanation of the ground covered in 
the following pages and the way in which the subject has been 
presented. The ordinary explanations and discussions of 
static electricity and other matters of theoretical interest only, 
such as are found in any high school physics, have been avoid¬ 
ed. The aim has been to make the book practical and utili¬ 
tarian throughout, with the sole object and endeavor to 
make the subject matter such as will be of immediate use to 
the reader. 

In many parts of the book some particular piece of 
apparatus or very important division of the subject, such as 
Wiring, has been allowed only a few pages. The Authors 
anticipate possible criticism for the superficial manner in 
which some of the important points have been taken up, but 
it is admittedly impossible to cover thoroughly in detail, in 
one volume, all of the broad field touched upon. The idea 
has been to give the reader the most comprehensive, and at 
the same time the greatest, amount of useful knowledge in a 
given space. Care has been exercised to make the explana¬ 
tions clear and the text interesting. 

Friends and Associates are to be thanked for valuable 
and timely suggestions. The Authors feel deeply indebted 
to Mr. E. E. Boyer, Electrical Superintendent of the General 
Electric Company, Lynn, Mass., for his kind advice and 
thorough reading of the proofs and revision of many particu¬ 
lars of the subject matter. Thanks are also due the Inspection 
Department of the Associated Factory Mutual Fire Insurance 
Companies, for the use of several illustrations for the chapters 
on interior and exterior wiring, taken from the book of rules 
published by that department. 


3 


THE AUTHORS. 


CONTENTS 


CHAPTER I. 9 

Electricity: The Many Ways in Which the Flow 
of an Electric Current is Similar to the Circulation of 
Water in Pipes — A Simple Circuit — The Meaning 
of the Terms: Volt and Ampere. 

CHAPTER II . 17 

Magnetism: Permanent Magnets — Magnetism 

and Magnetic Laws — Earth’s Magnetism — Com¬ 
pass Needle. 

CHAPTER III . 26 

Electromagnets: Magnetic Field Around a Con¬ 
ductor Carrying Current -— Production of North and 
South Poles by a Solenoid. 

CHAPTER IV . 34 

Instruments : Galvanometer — Dynamometer — 
Ammeter: Hot Wire; Electro-Magnetic; Moving 
Coil — Voltmeters. 

CHAPTER V . 44 

Ohm’s Law: Explanations Showing Why This Law 
Holds True for Direct-Current Circuits — Resistance 

CHAPTER VI . 49 

Circuits and Resistance: Voltage Drop in Cir¬ 
cuits— Series and Parallel Circuits — Methods of 
Determining Resistance. 

CHAPTER VII. 57 

Energy : Power — Explanation of: Watt — Kilo¬ 
watt — Watt-Hour — Kilowatt-Hour. 


4 










Contents 


CHAPTER VIII. 60 

Heating: Equivalent Values of Electrical, Me¬ 
chanical and Thermal Units and the Interchangeabil¬ 
ity of Different Forms of Energy. 

CHAPTER IX . 65 

Generation (Mechanically) of Electricity: In¬ 
duction — Cutting of Lines of Force. 

CHAPTER X . 74 

Development of Generators or Dynamos: Ap¬ 
plications of Principles of Generation. 

CHAPTER XI . 79 

Armatures: Ring and Drum — Construction — 
Windings. 

CHAPTER XII . 98 

Fields and Field Frames: Reasons for Use of 
Electro Magnets in Preference to Permanent Mag¬ 
nets — Field Windings. 

CHAPTER XIII . 109 

Methods of Excitation: Self, and Separate — 
Shunt, Series and Compound — Field Windings: 
Direction of Windings for Proper Polarity — Am¬ 
pere Turns — Laws Relating to Magnetic Strength. 

CHAPTER XIV.116 

Voltage: Formulas — Rheostats — Effect of 
Resistance in Series with the Field Windings on the 
Voltage Being Maintained or Generated by a Dynamo 
— Effect of Resistance in Series with Armature. 

CHAPTER XV.122 

Electric Motors: Cause for Rotation. 

CHAPTER XVI.126 

Electric Motors (Continued): Armature Drop — 
Back Electro-Motive Force — Discussion of These 
Two Quantities and of the Fact That Their Sum 
Equals Line Voltage — Starting. 


5 











Contents 


CHAPTER XVII. 

Armature Reaction. 


133 


CHAPTER XVIII .138 

Elementary Ideas of Commutation. 


CHAPTER XIX .142 

Brushes: Shifting and Setting of Brushes — 
Reasons for Shifting Brushes Forward from Neutral 
in a Generator, and Backward from Neutral in a 
Motor — Sparking — Interpoles. 


CHAPTER XX .160 

Curves: How Curves are Plotted and Their Use¬ 
fulness— Saturation Curves—Characteristic Curves 
of Shunt, Series, and Compound Wound Generators 
and Applications for Which Each is Particularly- 
Adapted. 


CHAPTER XXI. 171 

Alternating Currents: Explanations of Alterna¬ 
ting Currents and Some of the Differences from 
Direct Current — Curves of Alternating Current — 
Single Phase — Polyphase — Two and Three-Phase 
Current — Delta and Y-Connection — Alternators. 


CHAPTER XXII.185 

Transformers: Development — Constant Poten¬ 
tial Type—Simple Alternating Current Transmission 
Line and Explanation Why and How Alternating 
Current can be Transformed — Reasons for Use of 
Alternating Current for Long Distance Transmission 
Work — Constant Current Transformers. 


CHAPTER XXIII.202 

Rectifiers: Mercury Arc Type — Constant Po¬ 
tential — Constant Current. 


6 









Contents 


CHAPTER XXIV ..210 

A. C. Motors and Converters: Induction Motors 
— Synchronous Motors — Rotary Converters. 


CHAPTER XXV .!.223 

Motor Characteristics: Characteristics of Shunt, 
Series, and Compound-Wound Direct-Current Motors 
— Characteristics of Alternating-Current Motors. 


CHAPTER XXVI .239 

Motor Drive: Transmission of Power — Mechani¬ 
cal — Electrical — Efficiency of Transmission — 
Adjustable Speed Motors — Applications to Which 
Each Kind of Motor is Especially Suited — Power 
Required for Machines. 


CHAPTER XXVII .267 

Output: Limitations — Efficiency — Tests — 
Fahrenheit and Centigrade Thermometers — Rat¬ 
ings — Guarantees. 


CHAPTER XXVIII.278 

Wires: Copper, Aluminum, etc. -— Weight — Re¬ 
sistance — Current-Carrying Capacity — The Wire 
Table. 


CHAPTER XXIX .290 

Protective Devices: Fuses — Circuit Breakers — 

Oil Switches. 


CHAPTER XXX.302 

Interior Wiring: Underwriters’ Rules — Methods 
— Materials. 


CHAPTER XXXI.321 

Exterior Wiring: Underwriters’ Rules — Trans¬ 
mission Lines. 











Contents 


CHAPTER XXXII .339 

Central Stations : Operation — Equipment — 
Switchboards — Storage Batteries — Their Uses. 

APPENDIX 1.356 

Definitions of the Fundamental Electrical 
Units. 


APPENDIX II.357 

English and Metric Measures. 


8 





A GENERAL TREATISE ON 

ELECTRICITY 

AND ELECTRICAL APPARATUS 

CHAPTER I 
ELECTRICITY 

The Many Ways in Which the Flow oj an Electric 
Current is Similar to the Circulation of Water 
in Pipes — A Simple Circuit — The Mean¬ 
ing of the Terms : Volt and Ampere. 

P IECES of sealing wax or amber briskly rubbed 
on flannel or woolen cloth acquire a peculiar 
property, and bits of wood fibre or paper will 
be attracted and stick to the amber or wax if it is 
held near them. 



Fig. 1.—Electrified Sealing Wax. 

This peculiar physical property of amber was 
first noticed by a Grecian philosopher in ancient 
times. Later on, in about the year 1600, Dr. 


9 





Electricity and Electrical Apparatus 

Gilbert, an English Scientist, discovered that other 
substances besides amber — such as glass, resin, 
and sulphur—also possessed a similar quality. He 
called these substances “ electrics,” and since that 
time, the name electricity has been used to denote 
the invisible agent or power, which causes this 
mysterious manifestation or action of the amber or 
sealing wax. 

It has often been said that we do not know 
exactly what electricity is. While to a certain ex¬ 
tent this may be true, we are so well acquainted 
with its actions under almost every condition that 
we lose sight of the fact that its exact nature still 
remains unknown. 

Rubbing the amber or sealing wax causes the 
change noted, and when produced in this way, the 
electricity is present in static* charges only, and does 
not flow in a continual current. For these reasons, 
when so produced, it is called static or frictional 
electricity. 

Sometimes the leather belts used to drive 
machinery may be a little loose and slip on the 
pulleys. The friction due to the slipping of such 
belts produces static electricity which manifests 
itself or discharges in the form of sparks if the hand 
or fingers are held near them. This discharge con¬ 
sists of a single spark, or series of sparks, and not of 
a continual stream. 

For this reason, static electricity is unsuitable 
for many commercial applications, also the amount 
obtainable by rubbing sealing wax is too small to 
be of practical value for such purposes as furnishing 
light, and operating electric motors similar to those 
used on electric cars or on machine tools. 

Friction and influence-machines are manu¬ 
factured which can produce greater quantities of 
static electricity than it is possible to obtain from 
the sealing wax or glass rod. Figure 2 illustrates 
* Static,— stationary; not in motion. 


10 


Electricity 


a Toepler-Holtz influence-machine from which heavy 
charges of static may be obtained. Large machines, 
similar to this, are often used by physicians for 
medical purposes. 



Fig. 2.—Toepler-Holtz Influence Machine. 

Electricity may also be produced by batteries. 
When obtained in this way, it flows in a continual 
stream, and is called “current electricity. ,, A 
simple cell may be made by partially filling an 
ordinary glass jar with a solution of sulphuric acid 
and water, in which must be suspended a piece 
of zinc and a piece of copper. If a wire is attached 
to each of these metals, and the ends connected 
together, a current of electricity will flow. If 
allowed to continue flowing for some little time, a 
noticeable eating away of the zinc plate will result. 
In other words, the zinc is being burned chemically, 
and it is this chemical action which gives the energy 
necessary to the production of the electrical current. 
A current so produced is said to be “chemically 
generated.” It should always be remembered that 
electricity is a form of energy, and for its production 
or generation an expenditure of energy is necessary. 
Thus, the length of time a cell like this could supply 


ll 


Electricity and Electrical Apparatus 

current would be limited by the size of the zinc 
plate and the amount of acid solution available. 

Another factor affecting the output of the cell 
is‘known as polarization, which consists of the 
formation of small bubbles of hydrogen gas on the 
copper plate, caused by the chemical action decom¬ 
posing the water into its constituents, hydrogen 
and oxygen. These bubbles decrease the power of 
the cell. However, this action takes place only 
while the cell is active, and when it is not in use, 
part of these bubbles disappear and the cell regains 
some of its lost strength. 

Polarization causes many cells or batteries — 
such as the common dry battery—to “run down” if 
left connected to a circuit and used for long periods. 
Thus, “open circuit ” batteries, when not in use, 
should be disconnected in such a way as to prevent 
the flow of current. 

There are several different types of open circuit 
batteries on the market, working on the same prin¬ 
ciple as the above described simple cell, but using 
various metals for the plates and different solutions; 
in some cases a special chemical is added to the 
solution to reduce the polarizing action to a minimum. 

The ordinary dry cell or battery, as indicated 
by the name, is apparently without any acid solu¬ 
tion; one plate is a carbon stick in the center, and 
the other is a sheet of zinc made in a hollow cylin¬ 
drical shape, forming the vessel in which the other 
parts of the cell are placed. Care must be taken 
to keep the carbon and zinc from coming in contact 
with each other. The space between the plates is 
packed with a mixture of powdered carbon, man¬ 
ganese dioxide, and some absorbent material, like 
sawdust. This combined mixture is saturated with 
a solution of sal-ammoniac (ammonium chloride). 

Another class of batteries is known as “Closed 
Circuit,” and these are often used where current 
is required for long periods of time. They differ 


12 


Electricity 


from the open circuit batteries in respect to the 
duration of their current-furnishing power. 

The Gravity Battery is the foremost of this 
class and the only one worthy of mention. It con¬ 
sists of a “ crowfoot ” of zinc placed in the top of a 
jar containing a solution of copper sulphate or blue- 
stone. In the bottom of the jar is the copper plate. 

Open circuit batteries are used for door-bells, 
and places where current is necessary only for short 
intervals. Dry batteries are a special form of open 
circuit batteries, being utilized where convenience 
is desired. Closed circuit cells are suitable where 
currents are required over long periods of time. 
The gravity cell has been widely utilized in tele¬ 
graph work, but are now being superseded by motor- 
generator sets. 

Both open and closed circuit types are called 
primary batteries, as they are the primary or 
original source of the current. Storage batteries, 
sometimes known as accumulators or secondary 
batteries, are taken up later. 

Currents from primary batteries are suitable 
for purposes where only small amounts of electricity 
are required. If, however, several ordinary in¬ 
candescent lamps are to be lighted, or a large 
electric motor operated, it would require very many 
of these cells or batteries to furnish a sufficient 
amount of electricity. As this would make an ex¬ 
tremely unhandy and expensive arrangement, bat¬ 
teries are seldom used for this purpose. Instead, 
special machinery is installed in electric lighting 
plants or central stations, which generate an electric 
current large enough to light several thousand lamps 
or operate large motors. These machines, called 
dynamos or generators, are driven by steam or gas 
engines, turbines or water wheels. Current thus pro¬ 
duced is said to be mechanically generated. 

In the study of electricity it is well first to be¬ 
come familiar with electric currents themselves and 


13 


Electricity and Electrical Apparatus 

their action under various circumstances, before 
we proceed to the study of the machines and prin¬ 
ciples by which electricity is produced or generated. 

In many ways the flow of electricity can be 
likened to the circulation of water in pipes. The 
transmitting or passing of an electric current through 
wires— such as are stretched along the street, sus¬ 
pended from poles — is very similar to a stream of 
water flowing through a level pipe. To make the 
water flow through the pipe it is necessary to have 
a pump or some means whereby we can get a 
pressure to force the water along. This pressure is 
necessary to overcome the friction which the water 
meets in flowing through the pipe. 

In Fig. 3, if the faucet is closed and water is 
poured into the left-hand reservoir, it will flow to 



tubes is the same. As long as there is a difference 
in level, or in other words, a difference of pressure, 
the water will flow. 

Fig. 4 represents a stream of water passing or 
circulating around through a system of pipes. The 
water leaves the pump at a certain pressure, which 
causes it to flow through the pipes, whence it 
emerges, coming into contact with the water-wheel 
and losing the larger part of its pressure in turn¬ 
ing the wheel. It then falls into the tank and 
returning to the pump by suction through the 


14 





















Electricity 


bottom pipe, is again given a fresh impulse, and 
passes out from the pump along the same course. 



Fig. 4.— Hydraulic Analogy of Electric Circuit. 


The quantity of water flowing in the pipe, may 
be determined.by ascertaining the number of gallons 
that pass a given point in the pipe during a certain 
period of time. This can be done by placing a water 
meter in the pipe line as shown, or if the pressure is 
known, by measuring the cross section of the stream. 

The water is being delivered from the pump at 
a certain pressure, measured and expressed as so 
many pounds per square inch; while the water in 
the bottom pipe is under only a small pressure due 
to suction. In other words, there is a difference in 
pressure between the outlet and inlet of the pump, 
and the water will continue to flow only so long as 
this difference in pressure exists. In like manner an 
electric current will not flow of its own accord. It 
requires a force or pressure to overcome the resist¬ 
ance offered to its passage as it flows through the 
wire. Electricity is much like a weightless fluid — 
flowing or being conducted along or through the 
solid wire from particle to particle. 

There are many striking similarities in Fig. 5, 
which represents the production and utilization of 
electricity. The dynamo from which the electric 
potential is generated is shown on the left by a 
circle. A battery would also serve the purpose. In 
the ordinary case, the dynamo is, of course, located 
in the central station or power house and the current 


15 







Electricity and Electrical Apparatus 

is generated at a certain voltage or pressure, passed 
out along the wires in the streets to the customer's 



Fig. 5. — A Simple Electric Circuit. 


premises and thence into the house where the 
electric current is to be used. The pressure causes 
it to flow through the lamps or motors and to do 
its work, after which it returns by means of the 
second or bottom wire to the dynamo, where the 
pressure is again supplied’ and the current flows out 
over the same path. 

As in the pump and pipes, there is a difference 
in pressure between the two wires. This pressure 
is not measured in pounds per square inch as in the 
case of the water, but in another unit called the volt,* 
and on an ordinary circuit, where there are incan¬ 
descent lamps inserted, the pressure required to 
force the proper amount of electricity through a 
lamp filament is 110 volts. As the current flows 
the filament is heated, becomes incandescent and 
gives out light. 

Ampere* is the unit of current. If we know the 
number of amperes, it at once gives us an idea of 
the volume of the current, just as a clear idea of the 
size of a stream of water is gained when the cross 
section of the pipe is ascertained. 

* Definitions and values of the electrical units, Volt, 
Ampere, and Ohm, will be found in Appendix I. 


10 





CHAPTER II 


MAGNETISM 



Permanent Magnets —Magnetism and Magnetic Laws 
— Earth’s Magnetism — Compass Needle. 

If a horseshoe magnet is brought near a small 
piece of iron, it will attract and hold the iron 
as shown in Fig. 6. Also, steel pens, iron nails, 
and many other articles can be easily 
picked up, and, if the magnet is pow¬ 
erful, such substances as nickel and 
cobalt are noticeably attracted. We 
say this power is magnetism and the 
substances so affected are “magnetic.” 
On the other hand a copper cent, a 
silver dime, or a small piece of brass 
are unaffected -— hence such substan- 
Fig.6— Horse- ce s are “non-magnetic.” 

and°“Ke A e G per T ” Magnetism and magnets play a 
very important part in the commer¬ 
cial generation and utilization of electricity, being 
widely used in the various kinds of electrical 
machinery. In order to understand more clear¬ 
ly their action, let us perform the following 
experiment: — Lay a horseshoe magnet on a 
table. On top of the magnet place a piece of glass 
(heavy stiff paper will serve the purpose, if glass 
is not available). If fine iron filings are sprinkled 
oyer the glass and at the same time it is tapped 
lightly with a pencil, it will be seen that the filings 
do not fall haphazard but form along certain definite, 
curving lines, such as can be seen in Fig. 7. 


17 




Electricity and Electrical Apparatus 



Fig. 7.— Distribution of Iron Filings showing Mag¬ 
netic Field of a Horseshoe Magnet. 


Again, if a small bar magnet is substituted for 
the horseshoe magnet, we find the iron filings will 
distribute themselves as illustrated in Fig. 8. 



Fig. 8. — Magnetic Field of a Bar Magnet. 


18 

















Magnetism 


If both the horseshoe and bar magnets are held 
in an upright position beneath the glass, with only 
the poles or pole coming in contact with the glass, 
the resulting figures given by the filings will be as 
illustrated in Figures 9 and 10. 



Fig. 9.—Magnetic Field around the Poles of a 
Horseshoe Magnet. 


' v '* 


1 


: ' 

vV£*‘. • *v iv 

• r > V , -rSS--' 

: %‘J*r*^* '*-■ V vf 1 


> ' - . 



Fig. 10.—Magnetic Field around One Pole of a 
Bar Magnet. 


19 






Electricity and Electrical Apparatus 


Naturally the question arises: What makes 
the filings distribute themselves in such a manner? 
The space near any magnet is pervaded with mag¬ 
netic lines of force that are invisible, and in the 
direction or along the lines mapped out by the 
arrangement of the iron filings. This is called the 
magnetic field. While it is true that these lines 
of force are invisible, so much has been learned of 
them and their action under different circumstances, 
that we even go so far as to count them and deter¬ 
mine the number proceeding from a magnet. The 
strength of a magnetic pole is based on and depends 
upon the number of these lines. The more of these 
lines there are, the stronger the magnet, or in other 
words, the more powerful it will be. These lines 
of force emerge from one side of the magnet and 
enter the opposite side. The place where they 
emerge from the magnet is called the north pole, 
and where they enter or go into the magnet is called 
the south pole. Each line of force is a complete 
ring or link, and the curving path followed in going 
through the air from the north to the south pole can 
be explained as follows: The natural tendency of 
the lines of force is to travel by the path of least 
resistance, and through the air a straight line would 
represent the shortest, and consequently the path 
allowing the easiest, flow. But there is a repulsion 
between the individual lines of force flowing in the 
same general direction nearly parallel. This re¬ 
pulsion forces the lines outwards, giving them the 
curved shapes as illustrated. 

If two north poles of two bar magnets are 
brought near together —- each magnet being freely 
suspended by a string — they will repel each other; 
and if two south poles are brought near together, 
they will likewise repel each other. On the other • 
hand, if a north pole of one magnet is brought near 
the south pole of another magnet, they will attract 
each other. 


20 


Magnetism 


The cause for this repulsion and attraction of 
like and unlike poles will be more easily understood 
by the following: 

Place two bar magnets on the table with their 
north poles pointing towards each other and on 
them lay a piece of glass. Iron filings, sprinkled 
on the glass, will distribute themselves as in Fig. 11. 



Fig. 11.—Magnetic Field between North Poles of 
Two Bar Magnets. 


If the magnets are reversed, so that the two south 
poles point towards each other and the experiment 
is repeated, the filings assume similar shape, as will 
be seen in Fig. 12. After emerging from the north 



Fig. 12.—Magnetic Field between South Poles of 
Two Bar Magnets. 


21 












Electricity and Electrical Apparatus 

poles of the magnets in Fig. 11, the lines of force 
are practically parallel for some distance, and there 
is a resulting repulsion. 

On the other hand, if we place the two magnets 
with a north pole and a south pole towards each 
other, the lines of force, shown by the arrange¬ 
ment of the iron filings, will be quite different, as 
illustrated by Fig. 13. These lines of force emerge 



Fig. 13. —Magnetic Field between a North and a 
South Pole. 

from the north pole of one magnet and enter the 
south pole of the other. Since the lines tend to 
flow along the shortest path, they try to shorten 
themselves, just as rubber bands will do when 
stretched, and thus the poles are attracted toward 
each other. At the same time the lines repel each 
other, and are forced apart until the pull on each 
individual line balances the repulsion. This re¬ 
pulsion and contracting action can be plainly seen 
in Fig. 13. 

If a bar magnet, Fig. 14, is cut in halves, we 
have not obtained two magnets with one pole each, 
but both magnets have two poles. How does this 
happen? Because the lines of force which entered 
the south pole of the original magnet must flow 
through it, and as they emerge where the bar was 
cut in two, they form a north pole at that point. 


22 




Magnetism 


Iron is a magnetic material and allows the lines 
of force to pass with much less resistance than air. 
Thus, if a piece of iron is brought near a magnet the 




Fig. 14. —Effect of Cutting a Bar Magnet in Halves. 

lines of force will be distorted, as shown in Fig. 15, 
and on account of the tendency of lines of force to 
shorten themselves, the piece of iron will be attracted 
to the magnet. 

This could be explained in a different way as 
follows: Referring to Fig. 15; as the bar magnet 
is brought near to the piece of iron, some of the lines 
of force from the north pole of the magnet will enter 
the piece of iron, pass partly through it. and emerge 
from the opposite side. The passing of these lines 
of force through the piece of iron causes it to act as 


23 













Electricity and Electrical Apparatus 

a magnet. The side nearest the north pole of the 
magnet becomes a south pole as lines of force are 
entering at this point. The magnetism in the piece 
of iron is said to be induced. Now by the law of 
magnetism, where unlike poles attract each other, 
the north pole of a bar magnet will attract the south 
pole of the iron or induced magnet, and in this way 
the magnet is enabled to pick up the piece of iron 
which has become a magnet. 

Many years ago it was discovered that the earth 
itself is a great magnet, and that the north and south 
magnetic poles are not very far from the north and 
south geographical poles. 



Fig. 15. —Effect of Iron in Distorting the Field 
of a Bar Magnet. 

On this account, if a bar magnet is freely sus¬ 
pended by a string it will set itself in a certain 
direction, one pole being attracted towards the 
north pole of the earth and the other toward the 
south magnetic pole of the earth. 

There is a peculiar distinction concerning these 
so-called north or south poles of magnets that should 
be thoroughly understood. If a bar magnet is 
suspended, the end pointing northwards is in reality 


24 


Magnetism 


a north-seeking pole and as it is attracted by the 
north pole of the earth it must necessarily be of 
opposite polarity, but ordinarily when a north pole 
is spoken of we refer to the north-seeking pole or 
the one from which the lines of force emerge from 
the magnet. 

A compass needle is merely a light bar magnet 
that will turn easily on a pivot and therefore always 
points in a north and south direction, if not dis¬ 
turbed by local influences. 


25 


CHAPTER HI 
ELECTROMAGNETS 


Magnetic Field Arourid'a Conductor Carrying Current 
— Production of North and South Poles by a 
Solenoid. 




Under ordinary conditions, 
the flow of current through or 
along a wire will not change its 
external appearance. Yet when 
current flows, there are many 
ways in which it can be ascer¬ 
tained that several changes take 
place, both in the wire and in the 
space surrounding it. 

Over a small compass needle, 
pointing north and south, hold 
a wire parallel to it. Then, 
if the ends of the wire are 
connected to a battery or source 
of continuously - flowing current, 
the needle will suddenly be de¬ 
flected, or turned out of its north 
and south direction. 

If the ends of the wire which 
go to the terminals of the battery 
are changed, so as to reverse the 
direction of the current through 
the wire, the deflection of the 
needle will be in the opposite direc¬ 
tion. 

A small current will cause a 
needle to be deflected through a small angle. How¬ 
ever, if more batteries are used and a stronger cur- 



Figure 16. — Compass 
Needle, showing Ef¬ 
fect of the Electric 
Current. 


26 













Electromagnets 


rent obtained, the deflection of the needle will 
be increased. The stronger the current—that is, 
the more amperes that are flowing — the greater 
will be the deflection of the needle. A large cur¬ 
rent of a great many amperes will cause the 
needle to stand at nearly right angles to the wire, 
or in other words, point very nearly east and west. 

Suppose we take a second compass needle and 
place it directly above the wire: While the current 
flows it will be found that this second needle will 
be deflected in the opposite direction to one below 
the wire. Of course, it is the action or effect from 
the current in the wire which causes the needle to 
be deflected. This can be made clearer by the 
following experiment: 

Take a piece of heavy paper or glass, through 
the center of which a hole has been bored, and through 



A No Current Flowing 
B Current Flowing away from Observer 
C Current Flowing up to Observer 

Fig. 17. —Compass Needles Deflected by the Current 
in the Conductor. 

Note that a circle with a dot in the center represents a conductor 
with current flowing toward the observer, as though the dot were the 
point of an arrow. A circle with a cross represents a conductor with 
current flowing downward away from the observer, the cross indicat¬ 
ing the feathered shaft of an arrow as seen from behind. 


27 


Electricity and Electrical Apparatus 

this pass a vertical wire. If the ends of wire are 
connected as before to several dry cells of sufficient 
strength to give a current of 20 amperes, and four 
small compass needles are placed around the wire 
at equal intervals from each other — as shown in 
Fig. 17 — it will be found that all four needles are 
deflected differently. All four needles point in 
different directions — approximately 90° apart. 

If the ends of the wire going to the battery are 
exchanged, so as to reverse the direction of the flow 
of current through the wire, it will be noted that 
each of the four needles changes its position, or in 
other words, the direction in which the needles are 
deflected will be reversed. 

Now if the four compass needles are removed 
and iron filings are sprinkled about the wire, it will 
be noticed that they distribute themselves in a 
series of concentric circles around the wire. A still 
better demonstration of the experiment may be 
had by slightly tapping or jarring the glass, as this, 
of course, enables the filings to arrange themselves 
more easily. 



28 








Electromagnets 

These whirls, or circular lines of force, always 
proceed or rotate in'a certain direction with relation 
to the flow of current in the wire. An easily remem¬ 
bered rule to determine the direction of these whirls 
is: “Take the right hand and lay the thumb along 
the wire pointing in the direction the current is 
flowing, then, if the fingers are partly closed, the 
fingertips will point out the direction of whirls 
produced.” 



Fig. 19.—Rule to Determine Direction of Magnetic 

Field around a Conductor Carrying Current. 

(Right Hand.) 

It will be seen that the whirls and flow of cur¬ 
rent bear the same relation to each other as do the 
threads and forward travel of an ordinary right- 
hand screw. These whirls, or circular lines of force, 
around a conductor carrying current are the same 
as the magnetic lines of force described in Chapter II. 
These whirls constitute a magnetic field around the 
conductor. The more amperes that flow in the 
wire, the greater the number of whirls produced — 
that is, the stronger the current, the stronger the 
magnetic field. 

If a piece of wire is wound in several complete 
turns or convolutions, as shown in Fig. 20, it is 


29 


Electricity and Electrical Apparatus 

sometimes called a “solenoid,” and as a current is 
passed through the wire, around the turns of the 
solenoid, there will be a number of magnetic lines 



Fig. 20.—Solenoid. 


or whirls produced. At first it would seem that 
these whirls would be as illustrated, but instead of 
merely flowing around each individual turn of the 
conductor, all of the lines of force combine with 
the result as shown in Fig. 21. 



Fig. 21.—Magnetic Flux in a Solenoid. 

In this way we can produce a magnet electri¬ 
cally. For instance: Suppose a round bar of iron 
is placed in the middle of a coil of wire through 
which a current is flowing, the iron will become a 
magnet (Fig. 22). If the end of the bar which pro¬ 
jects from the coil is bent over, we have a horseshoe 
magnet. To proceed a little further; suppose a 
second piece of U-shaped iron is inserted in the coil 
and allowed to come in contact with the first piece, 
a complete ring or circuit of iron will be produced. 
As the lines of force leave one piece of iron and enter 


30 








Electromagnets 


the other they have the effect of making a north 
and south pole, respectively. These attract each 
other very strongly, as will be found on trying this 
experiment with apparatus as shown. 



Fig. 22a.—Solenoid. 



Fig. 22b.—Solenoid show¬ 
ing Distortion of Field 
due to Iron Core. 


An instructive and interesting experiment can 
be performed with the apparatus shown in Fig. 24. 

Solenoid shown is free to rotate and as ends are 
dipping in the two parallel grooves of mercury, 
current flows irrespective of position assumed by 
coil in its rotation. One end will be a north pole, 
the opposite, a south. This can be traced out by 
the rules given. 


Fig. 23a. —Distribution 
of Lines of Force with 
U-shaped Iron. 




Fig. 23b. —Use of Two 
U-shaped Cores, Dem¬ 
onstrating Magnetic 
Attraction. 


However, if current is passed through the sole¬ 
noid, and the north pole of a permanent bar magnet 
held near, as shown, it will attract the end of the 
solenoid in which the south pole is formed. The 
solenoid can be rotated in this way. 

It is much easier to magnetize wrought iron 
than hardened steel. However, steel after being 
magnetized will retain a large part of the magnetism 


31 



























Electricity and Electrical Apparatus 

even after the magnetizing force is removed. Per¬ 
manent magnets, such as the small bar and horse¬ 
shoe magnets, are, for this reason, made of steel and 
hardened. This may be explained somewhat by 
the following theory of magnetism: 



Fig. 24. —Attraction between a Solenoid or Electro¬ 
magnet and a Permanent Magnet. 

Suppose we place a glass tube, full of pieces 
or chips of iron or steel, in the center of the solenoid 
shown in Fig. 20. As a small current is allowed to 
flow it will be noted that some of the pieces of metal 
that are comparatively free to move adjust them¬ 
selves along certain lines. As the current is in¬ 
creased and the magnetizing force becomes stronger, 
additional pieces of metal move into these lines. 
On inspection, it will be noted that these lines along 
which the pieces of metal arrange themselves are 
identical with those mapped out by the lines 
in Fig. 21, where the field from a solenoid is repre¬ 
sented. If the current is increased still further, 
more of the remaining metallic pieces are brought 
into the lines. If the tube is carefully removed from 
the solenoid, so as not to disturb the arrangement 


32 



Electromagnets 


of the metallic particles, it will be found to act as a 
magnet. If it is shaken, however, and the arrange¬ 
ment of the particles disturbed — causing them to 
fall in odd positions, in a confused mass — it will 
be found that the tube will no longer act as a magnet. 

Any body or piece of metal, such as iron or 
steel, is composed of a number of very small particles 
which are known as molecules. It has been 
demonstrated that these little particles or mole¬ 
cules can move to a certain extent with respect to 
each other and without affecting perceptibly the 
rigidity of the metal. In any piece of iron 
each of these molecules is a small magnet, but 
under ordinary conditions these inolecules neu¬ 
tralize each other. A magnetizing force, when 
applied, causes these small particles to turn and 
point in the same direction. If the force is suffi¬ 
ciently strong it will cause them all to turn and set 
themselves along the lines of force. As this is done, 
the molecules or small magnets no longer neutralize 
but act together as one big magnet. 

These molecules are more difficult to turn in 
hard steel than in soft iron. Thus, while the iron 
is easier to magnetize, the hardened steel retains 
its magnetic powers longer, as the position of the 
molecules cannot be disturbed as easily. 


33 


CHAPTER IV 

INSTRUMENTS 


Galvanometer — Dynamometer—A mmeter: HotWire; 

Electro-Magnetic; Moving Coil — Voltmeters. 

We measure time by means of watches. When 
coal is bought, for instance, by the ton, it is weighed 
or measured oh scales. When we speak of a large 
electric current or a small electric current, we refer 
to the number of amperes flowing. 

If merely an indication of the strength of 
current is desired, and not an exact measurement, 
an instrument shown in Figure 25 may be used. 



Fig. 25. —Tangent Galvanometer. 

This is called a “tangent galvanometer,” and works 
on the principle that a compass needle is deflected 
from a north and south direction by the magnetic 
lines or whirls produced by the current. It consists 
of a compass needle situated in the center of a 


34 




Instruments 


circular coil composed of several turns, the ends of 
which are connected to the binding posts on the 
right. 

The deflection of the needle is proportional to 
the strength of the current in the coil, and the num¬ 
ber of turns. Also if the diameter of the coil is 
changed, the deflection of the needle will be affected 
correspondingly. As the diameter is increased and 
the distance between the wires and the needle made 
greater, the deflection will be smaller for a given 
current, or, on the other hand, if the diameter of 
the coil is decreased, bringing the wires closer to the 
needle, the current will affect the needle more strong¬ 
ly and cause a greater deflection. 

In any one galvanometer, where the diameter 
of the coil and the number of turns are constant, 
the current regulates the deflections of the needle, 
and as the strength of the current changes the 
needle varies in certain fixed proportions. 

We have seen, from Chapter II, how magnets will 
attract or repel each other. Now, likewise, there 
is an attraction and repulsion between two currents 
flowing in parallel wires exerted through the mag¬ 
netic fields surrounding each wire. 


If . two parallel wires 



are carrying currents in the 
same direction, the lines of 
®)! {(®) ) ! j force combine, as illustrat¬ 
ive ^ _ ed in Fig. 26, and due to 

—-—*- the tendency of the lines 



Two Wires Carrying Cur¬ 
rent in the Same Direc¬ 
tion. 


The resultant mag¬ 
netic field when the two 


currents are flowing in opposite directions is indi¬ 
cated in Fig. 27. As the lines flow along, side by 
side in the space between the two wires, there is a 
repulsion which will tend to force the wires apart. 


35 







Electricity and Electrical Apparatus 

The number of lines of force is proportional to 
the current in the two wires; consequently, the 
strength of this attraction or repulsion depends 
upon and varies with the currents. 



Fig. 27. —Magnetic Field between Two Wires Carry¬ 
ing Current in Opposite Directions. 

Fig. 28 represents a device for demonstrating 
the repulsion or attraction between currents flowing 
in parallel wires. The square or rectangular coil, 
as shown, is suspended on a pivot at the top and is 
free to rotate. The ends of the coil dip into two 
parallel circular grooves in the base, which are 
filled with mercury. In this way, connection can 
be made and is not broken as the coil rotates. The 
half circular coil and the rectangular coil are con¬ 
nected in series. The current will flow up one side 
of the rectangular coil and down the other side. 
Hence, one side of the rectangular coil will be re¬ 
pelled and the other attracted by the current in 
the straight side of the semi-circular coil. 

Dynamometers are constructed on this principle 
and are used to indicate current strength. One 
common type consists of two coils, the inner, station¬ 
ary, and the outer, suspended at right angles to the 
first. As current passes through both coils, the 


36 


Instruments 



Fig. 28.—Attraction or Repulsion between Parallel 
Conductors Carrying Currents. 

outer coil will tend to set itself or move into a 
position parallel with the first or stationary coil. 
This movement is against a spring, hence if the de¬ 
flection is measured, the current strength can be 
determined, 



Fig. 29.—Electro-Dynamometer. 


37 
































Electricity and Electrical Apparatus 

In order to tell exactly the amount of current 
flowing, it is necessary to measure and determine 
the number of amperes. The instruments used 
for this purpose are generally' called ammeters, of 
which there are several types as follows: 

1. Hot Wire 

2. Electromagnetic 

3. Moving Coil 

Additional types are sometimes used on alternating- 
current circuits. These will be described later. In 
order to understand how an electric current can 
be measured with an ammeter, let us take up the 
different kinds and examine their construction: 

Wires are heated by the passage of an electric 
current, and the larger the current the greater the 
amount of this heating. Most metals expand as 
they are heated, this expansion being proportional 
to the temperature of the wire —- the higher the 
temperature, the greater the expansion. The hot¬ 
wire ammeter will be readily understood by re¬ 
ferring to diagram shown in Fig. 30. The leads 



Fig. 30. —Diagram of Hot-Wire Ammeter. 

from the external circuit are connected to the two 
terminals, T and T. Between these two ter¬ 
minals is stretched a platinum-silver wire. To 
the^middle of this platinum-silver wire ^attached 


38 




Instruments 


another fine wire which is passed or wrapped around 
the cylinder C and thence to spring S. 

If the current passes through the platinum- 
silver wire it becomes heated, and consequent ex¬ 
pansion allows the spring S to pull the wire W 
towards the left and thus rotate the cylinder C, 
causing pointer to move across the scale. As the 
expansion of the platinum-silver wire is proportional 
to the amount of current flowing, the movement of 
the pointer will be correspondingly affected, and 
if the scale is correctly divided, the pointer will 
indicate the number of amperes in the circuit. 

Hot-wire ammeters can be used on direct or 
alternating-current circuits equally well, and are 
especially adapted for high-frequency work in con¬ 
nection with wireless telegraph apparatus. 

The electromagnetic ammeter is quite simple, 
but because it lacks sensitiveness and accuracy, is 
not very widely used. If the 
ends from the coil in Fig. 31 
are connected to a battery or 
source of current, the plunger P 
will be sucked upwards. Like¬ 
wise in Fig. 32, the plunger will 
be pulled or attracted in a down¬ 
ward direction, due to the mag¬ 
netic attraction. This attraction 
depends on the strength of the 
current, which can be measured 
by the rotation of the attached 
Fig. 31 —Coil and pointer. This meter is suitable 
Plunger. f 0 r uge on e ^her alternating or 
direct-current circuits. More than .all others com¬ 
bined, the ones most used are the “Movable Coil” 
type of instrument. 

If we have a coil, as shown in Fig. 33, mounted 
on an iron cylinder, and pass a current through it, 
north and south poles will be produced. Now if 
this coil and cylinder are pivoted and free to rotate, 



39 








Electricity and Electrical Apparatus 

when placed in a magnetic field as in B, they will 
tend to turn in the direction indicated. If the coil 
were mounted on a light frame of wood or some 



Fig. 32- —Electro-Magnetic, or Coil-and-Plunger 
Type, Ammeter. 

similar material, instead of the iron cylinder, and 
then placed in the magnetic field, there would be a 
force, as before, tending to turn the coil — although 
not as strong as before. 

A moving-coil type ammeter consists essentially 

of a permanent magnet, between the jaws of which 



is a cylinder-shaped iron core. In the annular 
space between this iron cylinder and the poles of 


40 









Instruments 


the magnet, is a small rectangular coil wound on a 
light aluminum frame. This frame is held or 
supported by jewel bearings, allowing it to rotate 
in the space shown. The pointer is attached to the 
aluminum frame. A view of one of these instru¬ 
ments complete as manufactured by the General 
Electric Company is shown in Fig. 34. The parts 



Fig. 34.—G. E. Type D2 Moving Coil Ammeter. 

are shown in Figures 35 and 36. Fig. 37 illustrates 
the moving element in position in an ammeter made 
by the Weston Electrical Instrument Company. 



Fig. 35.—G. E. Type D2 Ammeter with Cover Removed. 


41 


Electricity and Electrical Apparatus 


Ammeters are always connected in series with 
the line as has been shown and the coils in ammeters 
are, generally speaking, of large wire and few turns. 
Ammeters, therefore, have very small resistance, 
and if connected across, rather than in series with 
the circuit, they will in most cases be ruined by the 
resulting short circuit and probably burn the man 
connecting it in circuit. The comparatively high 
voltage across the circuit is sufficient to force many 
times normal current through the low-resistance 
coils of the ammeter, burning and destroying the 
insulation, and ruining the instrument. 



Fig. 36. — G. E. Type D2 Ammeter with Cover and Scale 
Removed, Showing Mechanism and Permanent Magnet. 

Voltmeters, or instruments used to measure 
voltages, are connected directly across the circuit 
between the two main lines, on all circuits on which 
the potential does not exceed six or seven hundred 
volts. Voltmeters are constructed on exactly the 
same principles as ammeters, only the coils or wind¬ 
ings used are of many turns of fine wire and offer 
a high resistance to the flow of current. In this 
way, when they are connected across the circuit, 


42 



Instruments 


only a small current is allowed to flow through the 
instrument windings. Sometimes, on circuits where 
the voltage is high, additional resistance coils are 
connected in series with the voltmeter, thus keep¬ 
ing the current down. These are called multipliers, 
as the voltmeter indication has to be multiplied 
by a constant to reduce to volts, when they are 
used. 



Fig. 37. —View Showing Interior ofWeston Moving 
Coil Ammeter. 

In order that an instrument may indicate 
accurately, it is necessary that the moving element 
move very freely. If this-is not the case, and the 
meter is “sticky,” small changes of current or volt¬ 
age would not cause the needle to move correspond¬ 
ingly. The moving elements of high-grade instru¬ 
ments are pivoted and supported in jewel bearings, 
similar to the moving parts of watches. Sapphires 
and diamonds are used for this purpose. 

These delicate bearings must be protected from 
vibration, or severe jolting, to insure perfect ac¬ 
curacy of operation. 


43 


CHAPTER V 


OHM'S LAW 

Explanations Showing Why This Law Holds True 
for Direct-Current Circuits — Resistance. 

In the illustration, P represents an iron water- 
pipe and the distance from E to 0 is quite 



long, say for instance, a mile or so. EA represents 
a stand-pipe such as is used in many of our cities 
as a means of obtaining pressure on the Water Works 
System. 

The amount of water in gallons per minute 
obtained at 0 will be proportional to the height 
of water in the stand-pipe, that is, EA. If the 
level or top of water is increased from A to B, 
making the column or height of water twice as high, 
the pressure exerted will be doubled and cause twice 
as much water to flow through the pipe. The flow 
is proportional to the pressure. 

It is obvious that the amount of water emerg¬ 
ing from the pipe at 0 depends directly upon 


44 








Ohm’s Law 


the size of the pipe P. The larger the pipe, the 
more water will flow, or if the pipe be made smaller, 
the amount of water obtainable will be less. The 
flow is proportional to the size of pipe. 

The length of pipe will also affect the amount 
of water flowing; naturally, if the pipe is made 
longer it will offer more resistance to the flow and 
diminish the quantity issuing at 0. 

If the inside of the pipe is rough, it will offer 
more resistance to the flow of water than if it is 
smooth. A polished glass tube will let more water 
flow, with the same pressure, than a rough iron pipe 
of equal size. Thus, besides the pressure, the size 
and the length of a pipe, the flow of water through 
it is also affected by the interior condition of the 
pipe. 

To sum up the above observations, it might be 
said that the flow of water depends upon the pressure 
and inversely upon the resistance offered to its flow. 
The resistance is dependent upon the following three 
conditions. 

1. Size of Pipe. 

2. Length of Pipe. 

3. Material or Interior Condition of Pipe. 

The passage of an electric current along a wire 
has many striking similarities to the flow of water 
in a pipe and is governed, to a certain extent, by 
similar laws. The amount of current which will 
flow along a wire depends, as with water in a pipe, 
upon two things: pressure and resistance. The 
resistance depends on the following: 

1. Size of Wire. 

2. Length of Wire. 

3. Composition and Condition of Wire. 

Of course in electrical work where we desire to 
transmit power and large currents for lighting 
purposes, it is best to have wires and conductors 
made of a material which will be strong and at the 


45 


Electricity and Electrical Apparatus 

same time conduct or allow the electricity to flow 
as easily as possible. Wires made of copper are 
most commonly used, as this metal has a very low 
resistance. The resistance of copper is less than 
that of almost any other material except silver, 
which is only a slightly better conductor than 
copper; and of course, silver would be too expensive 
to use in ordinary commercial work. Aluminum 
is sometimes employed, but for transmission lines 
one disadvantage encountered is the difficulty of 
joining it together or soldering it. Nearly all metals 
are conductors — even the human body will con¬ 
duct current to a certain extent. The resistance 
of the different metals is taken up more fully later 
on. 

There are many substances which will not con¬ 
duct electricity, such as porcelain, marble, slate, 
glass, rubber, guttapercha and air. These are 
called non-conductors or insulators. For instance, 



Fig. 39 . —Nernst Lamp, Exterior View, and Diagram of 
Connections in the Lamp. 


dry wood is an insulator but wood soaked with 
ordinary water will carry a current, due to the 
moisture it contains. Again, the glower used in 
the Nernst Lamp, when cold, will not conduct 


46 


















Ohm’s Law 


electricity. This glower is usually made of a com¬ 
position consisting of yttrium, magnesium and other 
substances, and has first to be heated before it will 
conduct the electric current. It is thus a non¬ 
conductor when cold and a conductor when hot. 

As we have seen, the resistance a substance 
will offer to the flow of current depends on several 
conditions, such as size, length, material, tem¬ 
perature, etc. Although resistance is an abstract 
quantity, that is, one which cannot be seen or 
handled, nevertheless it can be easily measured 
and is expressed in a unit called the ohm.* If we 
say a certain wire has so many ohms resistance, a 
clear idea is at once gained of this property of the 
wire. Unless the resistance changes the tempera¬ 
ture of a wire, it is independent of the amount of 
current flowing through it, and is practically a 
fixed quantity. 

The current flowing in a wire or electric circuit 
is in accordance with the law which was discovered 
by Ohm, a German physicist. For direct-current 
circuits, Ohm’s law is expressed as follows: Current 
in amperes equals volts divided by ohms. That is to 
say, that the number of amperes or intensity of 
current flowing in an electric circuit is directly 
proportional to the electromotive force, and also 
depends inversely upon the resistance offered to 
its flow. If we use the symbols, /, E and R to 
represent respectively the intensity of the current 
(in amperes), the electromotive force (in volts), and 
the resistance in ohms, we can write Ohm’s law in 
any of the following three ways: 

,-*■ ,-f. 

Thus of the three quantities, E, I or R, if we know 
any two, the third can be found. 

* Definitions and values of the electrical units, Volt, 
Ampere, and Ohm, will be found in Appendix I. 


47 


Electricity and Electrical Apparatus 

Suppose we have a coil, as represented in Fig. 
40, of a certain number of turns of wire of such a 
size that the resistance will be 10 ohms. If we con¬ 
nect this coil across an electric light circuit where 



the pressure is 110 volts, the current that will flow, 
by Ohm’s law (Form 1), will be as follows: 

r E 110 

/ = ^ = -jQ- =11 amperes. 

On the other hand, if we know the resistance of the 
coil, and with an ammeter measure the current flow¬ 
ing, we can calculate the voltage on the terminals 
of the coil by using the second form of Ohm’s law: 

E = RI = 10 X 11 = HO volts. 

Again, if the voltage is measured with a voltmeter 
and the current measured with an ammeter, we can 
calculate the resistance of the coil by Form 3: 

p E 110 in . 

R = J = -yy- = 10 ohms. 


48 







CHAPTER VI 

CIRCUITS AND RESISTANCE 

Voltage Drop in Circuits — Series and Parallel 
Circuits — Methods of Determining Resistance. 

Ohm’s law will be made plainer by the follow¬ 
ing experiment with glass tubing as shown in Fig. 41. 

If the faucet is wide open and water is poured 
in on the left through the funnel so as to maintain a 
level at A, the column AE will exert a pressure 
proportional to its height. This pressure will cause 
the water to flow to the right. It will be found that 



the water will rise to different heights in the four 
vertical arms, as shown, each lower than the pre¬ 
ceding one to the left. This shows that as the 
water flows through the tube, it loses some of its 
pressure in overcoming the resistance, or in other 
words, there is a drop in pressure. 

Fig. 42 represents a pump circulating water 
through a piping system. The water emerges from 
the pump under a pressure of, for instance, 11 pounds 


49 























Electricity and Electrical Apparatus 

per square inch. As it flows through the pipes, 
the pressure gradually decreases until, as the water 
returns to the pump, it is under practically no 
pressure. In other words, if we were to measure 
the pressure on the water in about the middle of 
the pipe at such a point as M, we would find that 
the pressure would be about 5.5 lbs. A little further 
on, such as at N, we might find the pressure to be 
4.5 lbs., and again at B we might find the pressure to 
be 3.1 lbs. or even less. As the water flows through 
the pipes there is a gradual drop in pressure. 



It is similar in an electric circuit as represented 
in Fig. 43; the direct-current generator is delivering 
current to an ordinary 16-candle power carbon 
filament incandescent lamp from a distance. The 
resistance of the lamp filament is 220 ohms while 
the resistance of each of the wires AB and CD is 
5 ohms. It is necessary to have 110 volts pressure 
to force the necessary amount of current, \ ampere, 
through the filament of an incandescent lamp. If 
less current than this flows, the filament will not 
be heated to the proper brilliancy and the amount 
of light will be decreased. If the resistances, AB, 
BC and CD are in series, the total resistance through 
which the generator must force current will be the 
sum of these three (5 + 220 + 5 = 230) which is 
230 ohms. Now if we must have £ ampere flow 
in the circuit, the voltage necessary to force this 


50 












Circuits and Resistance 

amount of current through 230 ohms can be found 
as follows: 

E = RI = 230 Xi = 115 volts. 

In other words, the generator voltage or the pressure 
from A to D must be 115 volts. To force £ ampere 
through the 5 ohms, which is the resistance of the 
wire AB, will require a pressure of 2\ volts; like¬ 
wise 2\ volts will be required to force \ ampere 
through 5 ohms, the resistance of wire CD. 



Fig. 43. 


The three voltages from AB, BC and CD will 
be respectively 2\, 110 and 2\ volts and are called 
the voltage drop for each part of the circuit. The 
total sum of these drops will equal the generator 
voltage. This is true in all direct-current circuits. 
The sum of the drops from A to B and C to D is 
5 volts, which will be the voltage drop in the lines 
or the pressure loss in overcoming the resistance 
offered to flow of current by the wires AB and CD. 

While the generator is delivering 115 volts, 
there is only 110 volts being received or delivered 
to the incandescent lamp. 

In Fig. 44 a direct-current generator is repre¬ 
sented as delivering current to an ordinary carbon- 
filament incandescent lamp. Assume that the 
resistance of the wires AB and CD is negligible or 



Fig. 44. 


51 














Electricity and Electrical Apparatus 

in other words, so small as to be inappreciable. If 
the generator is running at a speed that will give 
a pressure of 110 volts, \ ampere will flow through 
the incandescent lamp, providing the resistance 
from B to C is 220 ohms. 

Suppose we connect a second lamp in circuit, as 
indicated in Fig. 45. Providing the pressure between 
B and C remains 110 volts, there will now be 
1 ampere flowing as indicated on the ammeter in 
the line. By connecting the two lamps as indicated 
we have given the current two paths in parallel. 
The resistance of each lamp is still 220 ohms, and 



110 volts, the pressure between B and C, is 
sufficient to force \ ampere through each lamp. 
Now while the resistance of each lamp is the same, 
the two lamps together have the same effect as 
though we had connected one lamp from B to C 
with a resistance of 110 ohms, that is, the effective 
resistance of the two lamps connected in parallel 
is one-half that of either lamp alone. By giving 
the current the two paths, we have made it just 
that much easier for the current to flow. 

Fig. 46 represents a lamp bank with four lamps 
in parallel, or, as it is often called, in multiple. 
There will be \ ampere flowing through each lamp, 
providing the pressure remains at 110 volts. The 
ammeter will now indicate 2 amperes. Thus, the 
effective or combined resistance of four lamps in 
parallel is even less than two lamps in parallel. 
This is proved by the fact that 110 volts cause 2 
amperes to flow in place of 1 ampere. As the cur¬ 
rent is increased, the resistance must be less. The 


52 







Circuits and Resistance 


more lamps connected in multiple, the more paths 
are offered for the flow of current and the resis¬ 
tance becomes less in proportion. 



Fig. 46.—Lamp Bank, with Four Lamps in Multiple. 

If connected in parallel, the resistance of two 
lamps is one-half as much as that of one lamp, and 
the resistance of four lamps is one-fourth that of 
one lamp, and so on. 

Suppose on the other hand, we connect two 
lamps, as shown in Fig. 47, in such a manner that 
the current passes first through one lamp then 
through the other. The resistance offered to the 
flow of current between the points B and C 
is twice as great as though only one lamp were con¬ 
nected, as in Fig. 44. If the resistance is twice as 
great and the voltage between B and C is 110 
volts, the current will be one-half as much as in 


Fig. 47.— Two Lamps in Series. 

the arrangement represented by Fig. 44. With the 
two lamps connected in series, the total resistance 
would be 440 ohms and the current \ ampere. When 
connected in this manner, the lamps are said to be 



53 
































Electricity and Electrical Apparatus 

in series. In a series circuit it will be noted that 
the same current flows in all parts. 

If we go a step further and connect four lamps 
in series between points B and C, the total 
resistance will be 880 ohms and the current can be 
calculated by dividing the voltage by the resistance, 
or in other words, 110-f-880 would be £ ampere 
which will flow. The resistance of four lamps is 
four times as great as that of one lamp, consequently, 
the current is one-fourth as much. 



Fig. 48. —Lamp Bank, with Four Lamps in Series. 


On trolley circuits where the usual voltage is 
550, five 110-volt lamps are sometimes connected in 
series — as there will be approximately one-fifth 
of the total drop in each lamp, or 110 volts, there 
will be i ampere flow, which is the proper current. 
If only one lamp was connected across 550 volt 
mains, five times normal current would flow, burn 
the filament and ruin the lamp. 

There are several methods of measuring the 
resistance of conductors or substances. One of the 
most widely used is known as the “Drop of Poten¬ 
tial ” method, and is a very convenient way in which 
to determine the resistances of coils or windings. 
The resistance can be determined by measuring the 
voltage necessary to force a certain current through 
the coil. The instruments necessary are an am¬ 
meter and a voltmeter. 


54 























Circuits and Resistance 

Another common method or instrument used 
to measure resistances is the slide-wire bridge, 
illustrated in Fig. 49, and may be more easily 



Fig. 49. —Slide-Wire Bridge. 

explained by reference to Fig. 50, which is a dia¬ 
gram of the ordinary Wheatstone bridge. The resist¬ 
ance of the coil R is known, and GS 1 and GS2 are 
two adjustable resistance coils. X is the unknown 
resistance. Current from the batteries flows to A, 
where it divides and flows through the two paths 
ABD and ACD in inverse proportion to their 
resistance. Now there is a certain potential at 
A, a certain drop in the two paths, and a certain 
resulting potential at D. There is a certain drop 
from A to D — as the portion of the current flows 
through the path ABD , there is a gradual drop to 
the potential at D. Likewise there is a gradual 
drop in potential in the path ACD from A to D. At 
certain points, such as C and B, the potential is 
equal, if the GS1 and GS2 have been adjusted. 



Fig. 50. —Wheatstone Bridge Diagram. 

Consequently if a galvanometer is connected between 
B and C, as the potential is the same, no current 
will tend to flow through the galvanometer and 
hence there will be no deflection of the needle. When 


55 







Electricity and Electrical Apparatus 

in this condition the ratio of GS 1 to R is equal to 
the ratio of GS2 to X. Expressed in proportion: 

GS1 :GS2::R:X 

GS1 _ R 
01 ' GS2 X 

If GS1, GS2 and R are known, X can be easily 
found, as 

Y GS2XR 
GS1 

While in effect the same as in Fig. 50, the slide- 
wire bridge is in reality shown diagrammatic ally in 
Fig. 51. The same reasoning applies as in Fig. 50. 



Fig. 51. —Diagram op Connections of the 
Slide-Wire Bridge. 


If the size of a wire is known, its resistance can 
be found in the wire table which is taken up later. 


56 



















CHAPTER VII 
ENERGY 

Power — Explanation of : Watt — Kilowatt — Watt- 
Hour — Kilowatt-Hour. 

If we wish to measure the power obtainable 
from a river or a waterfall it is necessary to know 
the cross section of the stream flowing and the 
pressure available at the water wheel. If the head 
of water is increased to twice its former height, the 
stream will exert a proportionately greater pressure 
on the water wheel, due to the increased velocity 
it will acquire in falling through the greater distance. 
If the cross section of stream is doubled the turning 
effort on the wheel will also be doubled. Hence 
the work done by the stream depends on both the 
amount of water flowing and the velocity or pressure 
with which it strikes the water wheel. 

Work is the expenditure of energy, and such 
actions as turning waterwheels or lifting weights 
represent work done. A unit called the foot-pound 
is the amount of work done when one pound is 
raised through a distance of one foot. Thus raising 
one-half pound through a distance of two feet would 
be one foot-pound. 

The formula is: 

Foot-pounds = weight (in lbs.) X distance (in ft.).. 
If a waterwheel is being turned, work is being 
done at a certain rate, that is, so many foof-pounds 
per second or so many foot-pounds per minute. The 
power or rate at which work is being done is ex¬ 
pressed in foot-pounds per minute or second or in 
the unit called Horsepower. The horsepower is 
equivalent to 550 foot-pounds a second or 33,000 
foot-pounds a minute. It was first used by James 


57 


Electricity and Electrical Apparatus 

Watt, who was made famous by his improvements 
of the steam engine. In his time the collieries and 
mines in England used horses for hauling and hoist¬ 
ing. When he substituted his engine, the inquiry 
was: “How many horses will it replace?” Or in 
other words: “How many horsepower will the 
engine deliver?” Thus a horsepower is supposed 
to represent the rate work is capable of being done 
by a very strong horse. The colliery horses were 
very large and strong and as the horsepower was 
based on their strength and capacity, it is above 
the power of the ordinary horse, for any appreciable 
length of time. 

By the same reasoning, the power derived from 
an electric circuit is dependent on the pressure or 
voltage, as well as the current or amperes. The 
unit of power is the watt (named in honor of James 
Watt) being one ampere flowing under a pressure 
of one volt. To obtain the watts or power in any 
direct-current electrical circuit, multiply the volts 
by the amperes, their product being the watts. 

To light an incandescent lamp requires a certain 
amount of power. The ordinary 16-candle power 
carbon lamps have a filament of 220 ohms resistance. 
If the lamp is placed on a 110-volt circuit, i ampere 
will flow through the filament. The watts = E X / 
= 110 Xi = 55. 

If two of these lamps are placed in multiple 
on the 110-volt circuit, the total current is one 
ampere and the watts = E X I = 110 XI = 110. 

On the other hand, suppose these two lamps 
are placed in series on a 220-volt circuit, the two 
filaments in series will offer a combined resistance 
of 2X220 or 440 ohms and the current is i ampere. 
The watts = 220Xi = 110, or, in other words, the 
watts are the same whether the lamps are in series 
or multiple — this is as it should be as the light 
and the energy expended or work done is the same 
in either case. 


58 


Energy 


Three ways to write formula for watts in a 
circuit are: 

W = IXE 

W = I XIR = PR (as E = ) 

T Tr E r, E 2 , T E N 

(“ 1 ~ R ) 

The prefix “kilo” means thousand, hence kilowatt 
means a thousand watts, often used for convenience, 
as the watt is too small for many calculations in 
practice. 

If one consumer uses or lights two incandescent 
lamps for two hours and another consumer uses four 
lamps for the same length of time, it is obvious that 
the latter has used twice as much electricity as the 
former, and should pay twice as much. Likewise, 
if one man uses two lamps for two hours and a 
second has two lamps lighted for four hours, the 
quantity used in the second case is twice as great 
as in the first. It is equally important, in charging 
a customer for the quantity of electricity consumed, 
to consider the length of time as well as the number 
of lamps or amount of current required. 

The unit commonly used and in which the 
familiar integrating wattmeters register on their 
dials, is the kilowatt-hour. It is the quantity of 
electricity represented by the use of one kilowatt 
for one hour. Other units sometimes used are watt- 
seconds, being one watt for one second, and watt- 
hour, being one watt for one hour. 

Suppose, for instance, a man had 10 incandes¬ 
cent lamps in his store, each taking \ ampere at 
110 volts. They would consume 55 watts per hour 
per lamp and 10 lamps would consume 550 watts. 
If the lamps were lighted 5 hours per night, they 
would consume 5 times 550 which would be 2,750 
watt-hours or 2.75 kilowatt hours. If this customer’s 
rate from the Lighting Company is 16 cents per 
kilowatt hour, his nightly bill would be 2.75 times 
.16 which would be 44 cents. 


59 


CHAPTER VIII 


HEATING 

Equivalent Values of Electrical, Mechanical and 
Thermal Units and the Interchangeability of 
Different Forms of Energy. 

If a boat could pass through the water with¬ 
out meeting with any resistance it would require 
no effort to keep it in motion, but a boat in the 
water, or a wagon on land, or any moving object, 
meets and has to overcome friction, or in other 
words, the resistance offered to its motion; For 
this reason, to move objects and keep them in 
motion requires expenditure of energy. When 
substances are rubbed together and there is friction, 
they become heated; this heat is the form or way 
in which the energy used in overcoming the friction 
is manifested. The rougher the substances and 
the more rapidly they are rubbed, the greater will 
be the quantity of heat produced, which is only 
natural, as more energy is used. 

As electricity flows along a wire from particle 
to particle, the resistance offered to its flow is 
similar to friction. In overcoming this resistance 
and maintaining the flow of current, energy is 
expended which causes the wire to become heated. 

It is self-evident that more energy will be re¬ 
quired to force a certain amount of current through 
a wire of high resistance than through one of low 
resistance. Thus the conductor of high resistance, 
on account of the greater amount of energy ex¬ 
pended, will be heated to a higher temperature than 
will the wire of low resistance. Thus the filaments 


60 


Heating 


of incandescent lamps are very small in diameter 
and made of carbon composition or of metal, such 
as tungsten, and have a very high resistance. The 
amount of energy expended in forcing current 
through the filaments is converted into heat, the 
quantity of which is sufficient to cause the filament 
to glow. 

If a wire, while comparatively cold, is con¬ 
nected to a source of current, the temperature will 
rise, rapidly at first, then slower and slower until 
finally the temperature reaches a constant ultimate 
figure. At this point, the heat is being dissipated 
by radiation at the same rate as received by the 
wire. Radiation of heat depends largely on the 
outer surface of the wire. The larger the wire, the 
cooler it will be with a certain current passing, due 
to the increased surface exposed. 

At first sight it might seem that the heat in a 
wire would be proportional to the current flowing, 
but it is in reality proportional to the square of the 
current, that is, to the current multiplied by itself. 
If the current flowing in a wire is doubled, the heat 
is four times as great. If the current is made three 
times as large, the heat is nine times as great. This 
can be proved mathematically by the fact that the 
heating in a wire depends upon the energy expended 
or the watts lost in a wire. The watts equal the 
product of the current flowing and the volts drop 
or pressure required to force the current through 
the wire, and can be expressed as follows: 

Watts = El = RI X / = RI 2 (as E = RI). 

This will be more readily understood if we 
consider an experiment with a number of cells. Sup¬ 
pose we have a cell from which current is be¬ 
ing passed through a coil. If we connect two 
cells in series, approximately twice as much current 
will be passed through the circuit. Now as twice 
as much current is flowing through each individual 
cell as in the first case, there will be four times as 


61 


Electricity and Electrical Apparatus 

much zinc consumed and the energy furnished the 
circuit will be four times as great while the current 
has only been doubled. The heating effect of the 
current flowing through the coil will likewise be four 
times as great. 

If a coil of wire is immersed in a tank of water, 
and a certain current passed through the coil for a 
definite period of time, the temperature of the water 
will rise a certain amount. If the current is doubled 
and allowed to flow for an equal period of time, a 
thermometer, placed in the water, will show that 
the heat is four times the original temperature. 

Platinum is very infusible, being practically 
unaffected by very high temperatures. For this 
reason, currents can be passed through a platinum 
wire until it is quite hot without melting or fusing 
it. Therefore, platinum wires are often used in 
blasting work for igniting the charges; or under 
water, torpedoes can be exploded at will by the 
operator from a distance away on land. Also steel 
rails are welded together by passing an electric 
current from one to the other and causing them to 
get red hot at their junction. Electric cooking 
devioes depend for their heating upon a coil of wire 
which is made red hot by the passage of an electric 
current. 

We, of course, read the temperature of the air 
in a room by thermometers, but this merely tells 
us how hot the air really is; it does not tell us how 
much heat there is. The quantity of heat depends 
on the size of the room as well as the temperature. 
Obviously if one room is twice as large as another 
it would require twice as much heat to keep the 
larger room warm in winter as would be required 
for the smaller. 

The unit by which heat is measured in this 
country is the British Thermal Unit, often abbrevia¬ 
ted and called B.T.U. It is the amount of heat 
required to raise one pound of water from 39 to 


62 


Heating 


40° on the Fahrenheit thermometer. It is approxi¬ 
mately the amount of heat necessary to raise one 
pound of water 1° Fahrenheit. In France or coun¬ 
tries where the Metric System of measurement is 
used, and water is measured, not in pounds but in 
kilograms, instead of using the Fahrenheit ther¬ 
mometer, the Centigrade thermometer is used and 
the unit of heat with the metric system is called the 
Calorie. It is the amount of heat necessary to raise 
a kilogram of water from 0 to 1°C. In this country, 
in recent years, the Centigrade thermometer, on 
account of its simplicity and other advantages over 
the Fahrenheit, has come into universal use in 
scientific and engineering work. They are both 
described later on. 

Thus summarizing, we see there are several 
kinds or forms of energy in which we are interested: 
To operate a motor requires electrical energy, and 
we obtain mechanical energy from the.motor. When 
wires are heated there is thermal energy, and as a 
cell supplies current we might say there is chemical 
energy being changed to electrical. 

We have said electricity is one form of energy, 
and mechanical motion or power represents energy, 
only in another form. Thus lifting weights and 
turning water wheels means the expenditure of 
mechanical energy, which is measured in foot-pounds. 
When incandescent lamps are lighted or motors 
operated it also means consuming power, or in 
this case, electrical energy, which is measured in 
watts. If both foot-pounds and watts, or horse¬ 
power and kilowatts, are merely units used to 
measure different forms of energy; if the different 
forms of energy can be changed or transformed one 
to the other—the units likewise must be capable 
of interchangeability. 

This is true, and experiments and calculations 
show the various units to have equivalent values as 
set forth in the accompanying table. 


63 


Electricity and Electrical Apparatus 


UNIT 

APPROXIMATE 
EQUIVALENT VALUES 

1 H. P. 

550 Foot-lbs. per Second 
i 33,000 Foot-lbs. per Minute 

) 746 Watts 

j .746 Kilowatts 

f 42.42 B. T. U. per Minute 

10.689 Calories per Minute 

1 K.W. 

1,000 Watts 
\ 1.34 Horsepower 

/ 56.84 B. T. U. per Minute 

\ 14.323 Calories per Minute 

f 737.3 Foot-lbs. per Second 

44,240 Foot-lbs. per Minute 

1B.T.U. | 

.252 Calories 

778 Foot-lbs. 

1 Calorie j 

3.97 B. T. U. 

3088 Foot-lbs. 


64 











CHAPTER IX 


GENERATION (Mechanically) OF 
ELECTRICITY 

Induction — Cutting of Lines of Force. 

To generate means to make, and a generator 
is something that produces or generates. This 
chapter deals with the principles applying to the 
mechanical generation of electricity, and the follow¬ 
ing chapters take up commercial generators which 
utilize these principles. 

As previously explained, a conductor carrying 
current is encircled by magnetic lines of force. In 



Fig. 52. —Interlinkage of Electric and Magnetic 
Circuits. 


other words the electric current and the lines of 
force are interlinked, just as two links of a chain are 


65 

















Electricity and Electrical Apparatus 


connected together. No matter how small, it is 
impossible for any electric current to flow without 
producing some of these magnetic whirls, while the 
larger the number of amperes, the greater the num¬ 
ber of lines produced, as the number of these inter¬ 
linkages is dependent upon the current strength. 
It will be noticed also that these magnetic lines are 



in a plane at right angles to the direction of flow of 
current. These observations might be summed up 
in the following: An electric current is always 
interlinked with magnetic lines of force and at 
right angles to them. These whirls are produced by 
the flowing current in a wire. On the other hand, 
an electric current can be produced and made to flow 
in the wire or circuit by the action of magnetic lines, 



under certain conditions. .To investigate this phe¬ 
nomenon and see to what limits this statement will 
hold, let us proceed as follows: Suppose the ends 
of a loop of wire are connected to a milli-ammeter, 
as shown in Fig. 53. If a strong bar magnet is 


66 

















Generation of Electricity 

plunged into the loop, the magnetic lines encircle the 
conductor and the ammeter needle will be suddenly 
deflected. As there is no connection to any outside 
source of power, we might draw the conclusion that 
as the lines encircle the conductor, a voltage is in¬ 
duced which causes the current to flow. To obtain 



good results the ammeter used must be very sensitive 
as the currents induced in this way are very small. 
In some cases where the magnet is weak or the 
moving element of the ammeter a little hard to move, 
it will be better to use a galvanometer. 

If the magnet has been plunged into the loop, 
until midway, half in and half out, and is then held 
in this position, it will be seen that the needle comes 
to rest and returns to zero almost instantly after the 
motion of the magnet is stopped. Then, if the 
magnet is quickly pulled out, the needle is again 



deflected, but in an opposite direction to that when 
the magnet was entering the loop. However, as 
before, the deflection lasts only so long as the magnet 
is passing the conductor. We might then further 


67 











Electricity and Electrical Apparatus 

state that while a current can be induced in the loop 
by means of magnetic flux, it is only while the num¬ 
ber of lines encircling the conductor are changing, 
either increasing or diminishing, that a voltage, and 
consequently a current, is induced. While the num¬ 
ber of lines is constant there is no effect. This will 
be made still plainer by the following discussion 
and experiment: 

If a stiff copper rod, with its ends connected by 
flexible leads to a galvanometer or an ammeter, is 



suddenly thrust between the jaws of a permanent 
horseshoe magnet, the movement will cause the rod 
to cut across the lines of force, and as it enters the 
central part of the magnet the electric circuit or coil 
of one turn has interlinked with the magnetic cir¬ 



cuit. Due to this interlinkage, or encircling of the 
copper by the lines of force, the current is caused to 
flow. But the needle of the ammeter is deflected or 


68 
























Generation of Electricity 

indicates a flow of current only so long as the number 
of the interlinkages is increasing — or in other words, 
as long as the conductor is cutting across the mag¬ 
netic field. 

After the rod passes the jaws of the magnet, the 
number of interlinkages becomes constant, and the 
galvanometer needle returns to zero. Again if the 
rod is quickly drawn out through the jaws, the needle 
will indicate the flowing of a current. And also if 
the motion of the rod is made more rapid, the de¬ 
flections of the needle become proportionally greater. 



The conclusion can be drawn that if there is a 
relative motion between a magnet and an electrical 
circuit, so as to cause a variation in the number of 
interlinkages, either increasing or decreasing their 
number, a voltage will be induced that will be pro¬ 
portional to the rate of change in the number of in¬ 
terlinkages. 

Either the magnet or the wire may be the moving 
body, or both might be moved. It has been ascer¬ 
tained by extensive experiments that for every 
100,000,000 interlinkages around a conductor per 
second, there is one volt induced. Thus if we know 
the strength of the field and the time required for 
the conductor to cut through or interlink with it, 
the induced voltage can be easily calculated. For 
instance, suppose it takes one second for the rod to 
pass between the jaws at a uniform rate. If there 


69 













Electricity and Electrical Apparatus 

are 50,000,000 lines of force crossing from the north 
to the south pole, and these are cut by the conductor, 
the interlinkages are 50,000,000, and one-half volt 
will be induced. If the resistance of the rod, am¬ 
meter and flexible leads is assumed to be one-half 
ohm, this voltage will cause one ampere to flow while 
the motion of the rod is taking place, as the voltage 
is induced only so long as the motion continues. 

As the rod is moved and the current flows, there 
is a force or reaction opposing its motion. If the 
magnetic field is strong and the conductor moved 
rapidly, this reaction will be quite perceptible. In 
moving the conductor and overcoming this force 
we do work, and the energy thus supplied is changed 
into electrical energy. 

The direction in which the induced current 
flows is always definite, and has ascertain fixed 
relation to the direction of the motion and the lines 
of force. 



Fig. 60. Fig. 61. 

Lines of Force passing around a Conductor in a Clockwise Direction 
Induce Current Toward the Observer, and Vice Versa. 

If lines of force are passed around a conductor 
which is at right angles to the plane of this paper, 
in a clockwise* direction, the voltage induced is 
such as will tend to make the current flow upwards 
towards the reader, and when whirls are in a counter¬ 
clockwise* direction, the induced current flows away 
from the observer or downwards through the paper. 

This relation can be expressed by the following: 
If the fingers of the left hand are partly closed, so as 

* Clockwise,— in the same direction as the motion of 
the hands of a clock. Counter-clockwise,— in the opposite 
direction. 


70 


Generation of Electricity 

to encircle the conductor, with the finger tips point¬ 
ing along the magnetic lines, the thumb points in 
the direction induced current will flow. This is 



Fig. 62. —Rule to Determine Direction of Induced 
Current. (Left Hand.) 



6 


Fig. 63. 



Fig. 66. 




Fig. 64. 


Fig. 67. 



Fig. 65. Fig. 68. 


71 






















Electricity and Electrical Apparatus 

similar to the rule given in Chapter III for the deter¬ 
mination of the direction of the whirls or lines pro¬ 
duced by a current, and where the right hand was 
used, while here the whirls might be said to induce 
the voltages and thus produce the current, and the 
left hand must be used. 

The following explanation will make plainer the 
somewhat special and very common application of 
this rule, where a conductor is being moved across 
a magnetic field and the greater part of the magnetic 
circuit is not shown. 

In Fig. 63 practically all of the lines are passing 
from right to left over the approaching conductor, 
in which as yet no voltage has been induced. 

In Fig. 64 the conductor has moved upwards, 
cutting through a certain number of the lines. 

To calculate the induced voltage it is only 
necessary to know the number of lines actually cut 
through and the time taken. Provided there is a 
complete circuit offering a path for the current, we 
can easily determine its direction of flow even 
though all of each magnetic line is not visible, as the 
portion in sight is sufficient. Applying the rule, 
with the left hand, as shown in Fig. 64, encircling 
the conductor, point the finger tips in the direction 
of the lines of force just cut — the thumb will in¬ 
dicate current flowing upwards towards the observer. 

After the conductor has passed through the 
field cutting all of the lines, suppose it is brought 
to rest and then drawn downwards through the field, 
as in Fig. 65. The left hand must be turned upside 
down in order to allow the finger tips to point in the 
direction of the lines just cut. The induced current 
will flow downwards away from the reader. In 
other words — if the direction of motion is re¬ 
versed, the induced current will flow in the opposite 
direction. 

Now let us suppose the field magnet poles are 
exchanged (see Fig. 66), and the conductor is again 


72 


Generation of Electricity 


approaching. In Fig. 67 the lines cut are going 
from left to right underneath the conductor, and as 
the left hand must always be used, to point the 
finger tips in this direction it is necessary to hold the 
hand in the position shown. It will be noted that 
the direction of the induced current here is opposite 
to that in Fig. 64, for although the motion is the 
same, the magnetism is in a different direction. 
Also after the conductor has passed through the 
field if it is again drawn downward the current in¬ 
duced will be as indicated by Fig. 68. 

From these figures it will be seen that a reversal 
in either the motion or the direction of the lines of 
force will cause, likewise, a reversal in the direction 
the induced voltage will tend to make current flow. 


73 


CHAPTER X 


DEVELOPMENT OF GENERATORS OR 
DYNAMOS 

Applications of Principles of Generation. 

The wire described in the previous lesson, Figs. 
57, 58 and 59, if moved rapidly back and forth, in 
the jaws of the magnet, would produce a current 
whose strength would be continually changing and 
the direction of which would alternate with each 
change in the motion. That is to say, the current 
would flow in the wire first in one direction and then 
in the other. 

Instead of moving the conductor back and forth 
in a reciprocating motion a much simpler and better 
construction would be to bend the wire around in a 
coil as shown in Fig. 69 and mounted in some way 



on a central axis or shaft so as to allow rotation 
as indicated by the arrow. Connect the ends with 
two rings mounted side by side but insulated from 
the shaft and each other. 


74 


























Development of Generators or Dynamos 

In the position shown in Fig. 69, the sides of the 
coil will be cutting lines of force as represented 
diagrammatically by Fig. 70. Suppose the coil be 
rotated in a clockwise direction viewed facing the 
rings, from the right-hand side of Fig. 69, and indi¬ 
cated by arrows in Fig. 70. By rule in Chapter IX, 
there will be an induced voltage tending to make 
current flow away from the observer in the top wire 
and towards the observer in the bottom one. 

There are two springs or brushes, one bearing 
against each of the collector rings. By this means 
current is lead to the outside circuit and the direction 
of its flow is indicated by the feathered arrows. 

A quarter of a revolution later the conductors 
or the sides of the coil will enter a neutral region and 
for an instant the motion of each conductor will be 



Fig. 71. Fig. 72. 


along and parallel to the lines of force. At this 
instant, illustrated in Figs. 71 and 72, the number 
of interlinkages is not being changed; the voltage 
induced is zero and therefore the current, unless in¬ 
fluenced by other means, is zero. As the motion 
continues, both A and B commence to cut across the 
lines of force, at first slowly, then more rapidly, until 
the rate of change of interlinkages becomes a maxi¬ 
mum again in the position shown in Figures 73 and 
74. This causes voltage to be induced proportionally, 
and consequently, a current. But this current flows 


75 




























Electricity and Electrical Apparatus 

in an opposite direction to that illustrated in the 
first case, Fig. 69, as A and B have changed places, 
or in other words are now cutting through the field 
in different directions. 

Thus with a half revolution the direction of 
flow of current has been reversed or alternated and 
with another half revolution it will again change, 
thus returning to the original direction. With every 
complete revolution there are two alternations or 
changes in the direction of the current flow. 



Fig. 73. Fig. 74. 


This is called an alternating current, and, as we 
will learn later, it is particularly adapted for long¬ 
distance transmission work and some power pur¬ 
poses. However, if used for electroplating, for 
instance, it would be of no value, as the particles 
deposited on a plate from the solution while the 
current flowed in one direction would be immedi¬ 
ately cast back into the solution during the next 
alternation. Thus one alternation would undo the 
work of the preceding one. A direct or contin¬ 
uously flowing current would be required in cases 
like this. For lighting and some other purposes 
also, direct current, as a rule, is superior. 

If the simple change is made to the machine 
described in Figures 69, 71 and 73, of substituting 
a split copper tube as illustrated in Figures 75, 77 
and 79 for the collector rings, a direct current 'will 


76 



















Development of Generators or Dynamos 

be supplied instead of alternating. This spilt tube 
should be so arranged as to turn with the coil as 
the rings did in the previous illustrations. As the 
rotation is taking place in the position illustrated 
in Figs. 75 and 76, there would be voltages induced 
in the conductors A and B tending to make current 
flow in the directions indicated by the arrows. 



In Fig. 77 the coil has turned through one- 
quarter of a revolution and at this instant the con¬ 
ductors A and B, instead of cutting through lines 
of force, are moving along in a direction parallel with 



them. Consequently, at this moment the number 
of interlinkages is not changing, no voltage being 
induced in the conductors A and B, and neglecting 


77 


















































Electricity and Electrical Apparatus 

other influences there will be no current flowing 
either in the conductors A and B or in the external 
circuit. As the coil turns through another quarter 
revolution to the position shown in Fig. 79, the num¬ 
ber of interlinkages will be changing and voltages 
will be induced in A and B accordingly, tending to 
make currents flow in A and B in the directions 
indicated by the feathered arrows. On comparing 
Figures 75 and 79, one peculiarity noticed is, that 
while the direction of flow of current in the external 



circuit remains the same, it has been reversed in 
conductors A and B. Thus the current in the ex¬ 
ternal circuit is direct, while in A and B there is 
flowing an alternating current. This is due to the 
split copper tube, and on account of its rectifying or 
commuting action it is called a commutator. 

In all commercial machines for generating 
electricity on these principles, there must be, essen¬ 
tially, two parts: A magnetic circuit or field, and 
an electric circuit, so arranged as to allow a relative 
motion which will cause a change in the number of 
interlinkages of the two. 


78 




























CHAPTER XI 
ARMATURES 

Ring and Drum — Construction — Windings. 

With the magnet and the rotating coil ami 
commutator, described in Figures 75, 77 and 79 in 
Chapter X, the voltage obtainable depends, of course, 
on the rate per second at which the lines of force are 
cut by the conductors. This rate of cutting depends 
on and is proportional to the strength of magnetic 
field, and the speed of rotation. As it is impracti¬ 
cable to have the field with more than a certain 
number of lines per square inch or centimeter, and 
also unwise to exceed a certain speed, the amount 



Fig. 81. ^—Simple Armature with Coil of Two Turns 
Connected in Series. 

of voltage obtainable with this device is limited. 
If we wish to obtain more voltage, a coil with two 
turns can be substituted for the one shown in the 
previous lesson. 

Thus with the arrangement shown in Fig. 81, 
we would obtain twice the voltage if the coil is 


79 












Electricity and Electrical Apparatus 

rotated at the same speed and the magnetic field has 
been maintained at a constant value. As the coil 
turns and cuts through the lines of magnetic force, 
a certain voltage will be induced in each of the four 
wires, tending to make current flow in the direction 
indicated by arrow-heads. These pressures in the 
four wires or two loops are all in such directions as 
to help each other and the resultant or sum will be 
twice as great as when there were only two wires or 
one complete loop. 

This process might be carried still further and 
a coil wound with three, four, or even more turns, 
the voltage obtainable being increased in proportion. 
As the voltage increases, more and more current 
will flow around the circuit, in time becoming so 



Fig. 82.—Two Coils Connected in Parallel. 


large that the wires in the rotating coil would be 
overheated. To overcome this excessive heating, 
it would be necessary to connect a second coil in 
parallel with the first. Then the two coils, being 
in multiple and of the same number of turns, would 
present a double path, and half of the current would 
flow through each part. The voltage of the second 
would be the equal to that of the first or original 
coil and as the two are connected in multiple, the 
voltage supplied by the machine to the outside line 
is the same as before. 


80 














Armatures 


Actual machines used in practice have com¬ 
paratively large numbers of coils in the winding of 
the moving element, each consisting of many turns. 
These are wound or placed on an iron core, which is 
mounted on a shaft, together with the commutator. 
The coils, core, commutator and shaft all revolve 
together, the complete rotating element being called 
the Armature. By varying the connections of the 
coils in the armature winding, different voltages 
and current-carrying capacities can be obtained. 



Fig. 83.—Coils for Western Electric Drum Armature, 
Ready for Placing in Slots. 

Two general forms of armatures, distinguished 
from each other by the shape of the core and the 
position in which the coils are wound or placed, are 
Drum and Ring. Drum armatures are more widely 
used, and one is illustrated in Figures 83, 84, 85, and 
86 . 

It would at first seem that the cheapest and 
easiest way to make a core would be by casting, but 
if made in one solid body there are excessive eddy 
currents induced in the iron and in this way there 
is considerable power lost and the core becomes 
overheated. 


81 



Electricity and Electrical Apparatus 

If any electrical-conducting material is moved 
across lines of magnetic force a voltage is induced 
in the material. As the coil in Figure 87 revolves, 
currents will flow in directions shown. This is true 



Ready for Winding. 

whether the coil be made of copper, iron or any other 
substance, so long as it is a conductor. In the same 
manner consider section of an armature core as 
shown in sketch 88, assuming it to be solid rather 



Fig. 85. — General Electric Drum Armature, 
Partly Wound. 

than consisting of punchings. As this section of the' 
core revolves, different voltages will be generated 
in A, B and C, as A cuts all the lines of force, B jj 
and C J of them. If, for example, X V of a volt is 


82 








Armatures 



Fig. 86. — General Electric Completed Drum Armature, 

with Commutator. 




83 













Electricity and Electrical Apparatus 

generated in C, there will be generated in B and 
in A Au volts, illustrated by Fig. 89a. 

If voltage in Section A is three times that in 
Section C, a current will flow in the iron as shown 
in sketch 89b. These currents cause heating and a 



Fig. 89a. 



Fig. 89b. 



Fig. 90. — Punching for Armature Core. 


consequent loss of energy. On account of their 
circular course these are called Eddy Currents 
and are practically eliminated by making the arma¬ 
ture cores of japanned punchings. The japanning 


84 







Armatures 


serves as an insulator or obstruction to the flow of 
eddy currents, except in each individual punching. 

Some of these discs, punched out on presses, 
are shown in Fig. 90. In small armatures they are 
often mounted directly on the shaft, as shown in 
Fig. 91. The discs fit against a shoulder on one end 
and after the proper number has been mounted on 
the shaft they are tightened in place by the nut shown 
on the right-hand side. It will be noted that there 
is a keyway in the shaft and a corresponding slot on 
the inside of the discs. A key is inserted in these 
openings and prevents the laminations from turning 



Fig. 91. — Cross-Section Drawing, Showing Construc¬ 
tion of Small Armature Core with Punchings 
Mounted Directly on Shaft. 

or slipping on the shaft. The outside of the discs are 
punched with a number of slots, and when the arma¬ 
ture core is complete, these constitute or form re¬ 
ceptacles in which the wires or sides of coils are 
placed. 

In early days smooth-cored armatures were used 
and the windings held in place principally by bind¬ 
ing wires. The advantages gained on modern 
armatures by placing the coils in the slots are numer¬ 
ous, foremost of which is the protection of the wires 
and coil insulation from mechanical injury and the 
more compact and stronger construction gained 
thereby. Very often channels of fibre or strips of 


85 











































































Electricity and Electrical Apparatus 


other insulating materials are placed in the slots 
between the coils and the core, preventing the 
chafing or cutting of the coil insulation by the 
rough edges of the laminations and the resulting 
short-circuits and possible grounding of the winding, 
as well as assuring greater reliability of operation 
and less danger to operators. 

As we increase the number of coils it is advisable 
to connect the coils to different commutator seg¬ 
ments. If a large number of coils were connected 
to one segment there would be a large current, and 
this would cause injurious sparking every time the 
segment passed the brush. This is taken up more 
fully under Commutation, later on. 

Thus, as the armature windings in actual 
machines consist of many coils, we see also com- 




Fig. 92. — A Commutator Bar. 


mutators consisting of many segments, and each 
segment must be thoroughly insulated both from 
the others and from the shaft or spider on which they 
are mounted. 

Besides being well insulated from each other, 
the segments must be held firmly in place, for if this 
is not done, one or more of them may become loose 
as the armature is rotated and thus give what is 
known as a “high bar.” the loose part or segment 
projecting above the others and with each revolution 
raising the brushes and throwing them away from 
the commutator surface, thus causing sparking and 
chattering of the brushes. Commutator construction 
involves careful mechanical work and is a very im¬ 
portant part of the machine. 

Figure 92 shows a commutator segment, 
which is pressed or punched out from thick bar- 


86 






Armatures 


copper. Mica is the most generally used substance 
for insulation in commutator work, and the most 
satisfactory form is that known as split mica, which 
is simply ordinary sheet mica split up into small 
pieces which are tested up for imperfections, breaks 
and metallic veins, substances which would injure 
the insulating quality. These small pieces are pasted 
together with shellac or an insulating compound or 
varnish and in this manner a sheet of large dimen¬ 
sions can be made up with much higher insulating 
qualities than that of mica found in its natural state. 


Z o*r*/*>arA*o/is 



Sp/der 


Fig. 93. — Cross Section of an Armature and Com¬ 
mutator Without Winding. Core Mounted on 
Spider. 

To build a commutator, the proper number of 
segments are assembled in ring shape, sheets of mica 
being used to separate the segments from each other. 
Then V-shaped rings of mica are pressed in the two 
grooves shown, and on top of these are fitted the 
lips of the end plates. As the two end plates are 
tightened or pulled together by screwing the nuts 
on the; bolts, it causes the segments to be firmly 
held in place. It will be noted that in the radial 


87 

































































































































Electricity and Electrical Apparatus 

lug, on each commutator segment, is a small groove 
or slot, into which the ends of the wires or con¬ 
ductors from the armature coils are soldered. 

In the larger machines there are a great many 
coils in the armature windings and a corresponding 
number of slots in the armature cores. To one un¬ 
familiar with the method of connecting the coils it 
may at first seem a very complicated process. How¬ 
ever, all armature windings depend on the same 
principles and methods of connection, and if a few 
of the more simple forms of windings are mastered 




Fig. 94. —Construction of Commutator. 

and the principles understood, the more intricate 
ones can be readily grasped. Windings on drum 
armatures can be divided into two general classes: 
Multiple or Lap windings, and Series or Wave 
windings. These two kinds of winding can be 
easily distinguished. 

But before going into the winding as a whole, 
there are two or three things about the individual 
coil that it will be well to consider. Fig. 95 shows 
one coil of a drum armature winding in a multipolar 
field. The two slots X and Y contain the sides of 


88 






















Armatures 


the coil and are separated from each other on the 
periphery of the armature by an angle « which is 
called the angular pitch or spread of the coil and 
should theoretically equal a pole pitch, represented 
by angle ft, which is the angle between the pole 
centers. On a bipolar machine the pole pitch would* 
be 180°, on a four-pole machine 90° and on a six- 
pole machine 60°, etc. In ordinary machines the 
angular pitch of the coils is just a little less than the 
pole pitch of the machine, as this shortens the end 



Fig. 95. — Sketch Showing Spread or Pitch of an 
Armature Coil. 

connections of the coils from slot to slot and saves 
copper. If the angular pitch of the coil is made too 
small, however, it will cause trouble in commutation 
and will not work efficiently. This will be seen 
more clearly in a later chapter, where commutation 
is taken up. 

The connection pitch of a winding may be de¬ 
fined as the number of commutator segments which 
are overlapped by the ends of a coil, or in other 
words, if all the commutator segments were num¬ 
bered consecutively 1,2, 3, etc., and the connection 


89 












Electricity and Electrical Apparatus 


pitch, say for instance, is 8, it would mean that the 
two leads from one coil would be connected to seg¬ 
ments 1 and 9. 

Fig. 96 represents the difference in coils such 
as would be used in multiple or lap windings and in 
•series or wave windings respectively. It will be 
noted in the multiple or lap windings in Fig. 97, the 
ends of the coils come back to adjacent segments 
of the commutator and the coils of such a winding 
lap over each other. In the series windings the 
coil ends instead of coming toward each other diverge 
and go to segments widely separated on the com¬ 
mutator, and, as will be seen in Fig. 97, the winding 



Fig. 96.—Armature Coils for Lap and Wave Windings. 


to a certain extent resembles a wave and the coils 
are all in series by the method of connection. 

Fig. 98 represents a bipolar winding with 16 
slots, 8 coils and 8 commutator segments. At first 
it may seem that the wires are criss-crossed back 
and forth in a haphazard manner. But with a 
clockwise rotation of the armature, as indicated by 
the arrow, let us see the direction currents will tend 
to flow in the different armature conductors, if 
leads were connected from the brushes B, B, to an 
external circuit. It will be seen that there are four 
conductors under each pole: 3, 4, 5, and 6 under the 
south pole, and 11, 12, 13 and 14 under the north 
pole. Each one of these coils in an actual winding 


90 

















Armatures 


might consist of many turns, although here only 
one turn is used for simplicity, and in tracing out 
the connections the solid lines represent the con¬ 
nections on the front of the armature from the com¬ 
mutator bars to the slots, while the dotted represent 
the connections on the back of the armature, or in 
other words, merely the rear end of the coils. 

By the rules given in previous pages, currents 
would tend to flow in 3, 4, 5, and 6, in a direction 



Fig. 97. —Winding Diagrams for Lap and Wave Wind¬ 
ings, with Forward and Backward Progression. 

toward the observer up through the paper, and in 
11, 12, 13 and 14 in a direction away from the ob¬ 
server. If we trace the relative connection between 
the coils and commutator, we will see that these 
induced voltages in the individual conductors on 
each side of the armature are all added together and 
help one another, so that the total voltage or pressure 
between the two brushes is the sum of all on that 
side. 


91 




Electricity and Electrical Apparatus 

Suppose the brushes B, B, are connected to an 
outside circuit and the generator is in operation. 
Let us trace out the direction of flow of current 
through the winding. Brush B x at the instant 



Fig. 98. — Simple Bipolar Drum Winding. 


shown is touching commutator segments 5 and 6, 
and the current would be entering the machine from 
the outside circuit at this point. On leaving the 
brush it would divide, part going to commutator 



Fig. 99. — Electrical Circuits in Bipolar 
Armature Winding. 

segment 5 and part going to commutator segment 
6. In other words, there will be two paths offered 
to the flow of current. We will take them up sepa¬ 
rately: 

After the current leaves commutator segment 
6, there are two wires from 6 which would conduct 


92 












Armatures 


current, one leading to conductor 16, the other 
leading to conductor 11. But if current was con¬ 
ducted to slot 16, down the slot and across the back 
of the armature, as indicated by dotted line, to con¬ 
ductor 9, up the slot and across to commutator seg¬ 
ment 5, as shown by solid line, it would merely be 
back to the brush again. In other words, the coil 
whose two sides are conductors 9 and 16 is short- 
circuited by the brush, and on that account we will 
consider that no current is flowing in this coil. 



Fig. 100. — Four-Pole Lap Winding. 


Again going back to segment 6, the current 
could take the other lead to conductor 11, by the 
dotted line to conductor 2, come up the armature 
to the front and down as indicated by the solid line, to 
commutator-segment 7, and from there into conductor 
13, down the*armature, across the back from 13 to 4, 
from conductor 4 to segment 8, from there to con¬ 
ductor 15, from conductor 15 to conductor 6, from 
conductor 6 to segment 1, from there to the outside 


93 




Electricity and Electrical Apparatus 

circuit via the positive brush B 2 ’, after leaving the 
machine the current goes on the outside circuit 
around through the lamps or other load we may 
have, after which it returns to the machine, entering 
as before by brush Bi, which is called the negative 
brush on a dynamo. 

On entering the machine through the brush 
B lf let us consider the other path offered to the 
current, or in other words, start with segment 5, 
from there to conductor 14, down by the dotted line 



Fig. 101. — Four-Pole Wave Winding. 


to conductor 7, from conductor 7 to segment 4, to 
conductor 12, to conductor 5, to segment 3, to con¬ 
ductor 10, to conductor 3, to segment 2, to positive 
brush and out to the line again. 

Tracing the direction of the current as it flows 
through the winding shows that between the two 
brushes there are two paths in multiple. The wind¬ 
ing might thus be said to have two circuits, and some¬ 
times an armature winding of this type is repre- 


94 



Armatures 


sented in sketches by a complete circular ring of 
winding with two taps at diametrically opposite 
points, the current entering at one and leaving at 
the other. 

In drum windings every conductor on the 
armature is active, cutting lines of force during each 



Fig. 102. — Ring-Wound Armature, as Used on Brush 
Arc-Light Generators. 


revolution, the only parts of the coil doing no 
useful work being the back connections and the 
leads from the slots to the commutator segments. 

In ring armatures the core is ring-shaped and 
supported on a spider, which is in turn mounted on 
the shaft. The windings are wound around the 
ring. A ring armature is shown in Fig. 102 and 


95 




Electricity and Electrical Apparatus 

diagrammatically in Fig. 103. The distribution of 
lines of force in a bipolar machine with ring arma¬ 
ture is shown in Fig. 104, and, as will be seen in the 
rotation of this armature, the conductors on the 
inside surface of the core will cut or interlink with 



Fig. 103. — Diagram of Ring Winding. 


practically no lines of force and consequently will 
do no useful work. Therefore, in a ring winding, 
in addition to the end connections on both back and 
front of the armature, we have the inside part of the 
coil inactive; and the resulting uneconomical use of 



Fig. 104.—Path of Magnetic Flux in 
Ring Armature. 

copper, and the difficulty of winding such a shaped 
core, are factors in preventing a low cost of production 
for ring-armature machines, and in limiting the field 
of their application. On the other hand, by their 
shape and the method of winding such armatures, 


96 
































Armatures 


the coils do not overlap or touch each other, making 
it much easier and cheaper to insulate the windings 
very effectively. For this reason this type of 
machine is used where a very high voltage is re¬ 
quired, such as for series arc-light circuits. 

In alternators, a very common method is to 
place the armature winding around in slots in punch- 
ings held by the frame. In these cases the arma¬ 
ture is stationary and the field poles revolve inside. 
This construction is explained in the next chapter. 


97 


CHAPTER XII 


FIELDS AND FIELD FRAMES 

Reasons for Use of Electro Magnets in Preference to 
Permanent Magnets — Field Windings. 

On account of their simplicity and cheapness 
“permanent” magnets are employed in many 
electrical measuring instruments and in magnetos, 
such as are used in motor boats and automobiles. 
However, it is difficult to make a magnet, the strength 
of which will not decrease in use, and in endeavoring 
to turn out magnets that will retain constant strength 
for several years, makers employ special steel alloys 
which are put through various hardening and arti¬ 
ficial ageing processes. In small sizes the magnets 
so produced will withstand a remarkable amount 
of hard usage, their magnetism remaining practi¬ 
cally unchanged during long periods of service. How¬ 
ever, in anything but very small sizes, it is commer¬ 
cially impossible to produce a satisfactory permanent 
magnet, and on this account it is universal practice 
to employ electro-magnets in practically all of the 
motors or dynamos used in every-day work. 

With the development of the electrical manu-. 
facturing industry, we find different makers employ¬ 
ing a large variety of shapes of field magnets and 
frames in the construction of dynamos and motors. 
A very common form in the earlier days of the in¬ 
dustry is shown in Figs. 105 and 106, the magnet 
system being an inverted horse shoe, or U-shape. 

Another form is shown in Figs. 107 and 108. 
This machine was manufactured by the Thomson- 
Houston Electric Company and was used for in- 


98 


Fields and Field Frames 

candescent and arc lighting purposes, the windings 
and frame being modified somewhat from the in¬ 
candescent form when used for arc lighting. 



Fig. 105.—Field Magnets and Frames on Early 
Bipolar Dynamos. 

The construction and relative position of the 
field poles and armature in Fig. 105, as can be 



Fig. 106. — Old Style Bipolar Dynamo. 


readily appreciated, left the armature unprotected 
to a large extent and in many cases exposed the 
windings to injury. To protect the, armature, in 


99 




Electricity and Electrical Apparatus 

the later forms of machines, a circular frame was 
brought around, enclosing the armature and poles 
at top, as shown in Fig. 109. The top section of the 



Fig. 107. — Early Form of Field Magnets Used in 
Thomson-Houston Machines. 


frame is made of non-magnetic material, such as 
brass, in order to eliminate as far as possible any 



Fig. 108. — Thomson-Houston Arc-Light Generator, 
Now Obsolete. 

magnetic leakage. While this was an improvement 
upon preceding designs, the outside frame, beyond 
serving for protection, was inactive and did no 


100 










Fields and Field Frames 

useful work, making the machine heavy for any 
given size. 

In the latest forms of bipolar machines brought 
out, there is a frame arrangement of field poles, 



Fig. 109. — Early Bipolar Dynamo with Frame 
to Protect Armature. 

as shown in Fig. 110. The outside frame, which 
protects the armature, serves as a part of the mag¬ 
netic circuit, and for a given size this type of a 
machine is lighter and more compact than that 



Fig. 110. — Modern Bipolar Field and Frame. 

shown in Fig. 109. It will be noted in the illustra¬ 
tion of this machine, that the end shields, which 
support the bearings, are shaped to protect the com¬ 
mutator and brush rigging, as much as is consistent 
with good ventilation. Also these end shields are 


101 




Electricity and Electrical Apparatus 

fastened to the frame or yoke by means of four bolts 
90° apart, permitting them to be rotated either 90° 
or 180° if desired, in order to prevent the spilling of 
oil from the bearings when the machine is to be 
mounted on the wall or suspended from the ceiling. 

The machines so far considered have had only 
two field poles, one north and one south, this being 



Fig. 111. —General Electric Form CQ Bipolar Dynamo. 


the most widely adapted form of construction for 
generators in the smaller sizes such as 1, 2, 3 and 5 
kilowatts. However, in many of the larger sized 
dynamos, four, six, eight or even more poles are 
employed, these being called multipolar. There 
are several reasons for this change in design and 
construction as the capacity of a machine is in- 


102 


Fields and Field Frames 

creased. Most of the smaller dynamos or motors 
are of the belted type and are operated at speeds 
varying from 1,200 to 3,600 R.P.M. 



Fig. 112. — CQ Machine Mounted on Post or Wall. 



Fig. 113. — Modern Four-Pole Field and Frame. 

But in the larger sizes, if the armatures were 
operated at such speeds, the centrifugal force and 


103 









Electricity and Electrical Apparatus 



consequent mechanical strains would cause them 
to fly to pieces. Generator shafts are sometimes 
coupled directly to the main engine or turbine shaft 
and in such cases the speed of the driven machine 
is naturally the same as that of the driver. Where 


104 


Fig. 114. — Fort Wayne Four-Pole Dynamo, Showing Parts Not Assembled. 



Fields and Field Frames 


Fig. 115. — Fort Wayne Six-Pole Generator. 

large Corliss engines are used, this means speeds as 
low as 80 or 60 R.P.M. Thus the engine equip¬ 
ment is sometimes another factor necessitating low 
speeds. 

As the speed of a generator is lowered, the 
armature conductors will cut fewer lines of force per 
second and the voltage decreases proportionally. 
To maintain the desired voltage the number of 
armature conductors might be increased, or the poles 
made larger and the strength of the field increased. 
This soon leads to very clumsy construction, how¬ 
ever, and bipolar machines in larger sizes are seldom 
built. 


105 



Electricity and Electrical Apparatus 



Fig. 116. — Fort Wayne Multipolar Revolving 
Field Generator. 

All of the machines so far considered, whether 
bipolar or multipolar, consist essentially of a cylin¬ 
drical armature rotating in a magnetic field, but it 
is immaterial, theoretically, whether it be the arma¬ 
ture or field magnets that are moved. The necessary 
condition is that there be a relative motion causing 
a change in the number of interlinkages. Fig. 116 
represents the external appearance of a machine in 
which the field magnets revolve while the armature 
winding is placed around the inside of the yoke and 


106 



Fields and Field Frames 



Fig. 117. — Field Structure for Fort Wayne 
Revolving Field Generator. 



Fig. 118. — Frame and Armature for Fort Wayne 
Revolving Field Generator.' 


107 





Electricity and Electrical Apparatus 

is stationary. Fig. 117 is a view of the rotating 
element and shows the two slip rings which the 
brushes press against, and by means of which the 
current passes through the field windings. Fig. 118 
shows the armature winding. 

In practically all direct-current machines the 
field magnets are stationary and surrounding the 
armature which rotates. With very few exceptions, 
alternators are now made with revolving field con¬ 
struction. 


108 


CHAPTER XIII 
METHODS OF EXCITATION 


Self, and Separate — Shunt, Series and Compound — 
Field Windings: Direction of Windings for 
Proper Polarity — Ampere Turns — Laws Re¬ 
lating to Magnetic Strength. 


While permanent magnets are well suited for 
some purposes the generators used in ordinary com¬ 
mercial work employ electro-magnets, and the cur¬ 
rent used to energize the windings of these electro¬ 
magnets is secured either from some external source 



Fig. 119. — Electrical Circuits — Shunt Machine. 


or from the machine itself. The first of these two 
methods is known as separately exciting the machine, 
the current being obtained from a separate generator 
which is called an Exciter, as the current it supplies 
is exciting the field magnets of the first machine. 
The second method is designated self excited, as the 
machine furnishes its own excitation. 

As a rule, alternators are separately excited. 
Direct-current machines may be either separately 
or self excited equally well. Figs. 119, 120 and 121 


109 


















Electricity and Electrical Apparatus 

illustrate three methods of connections, known as 
Shunt, Series and Compound, for self-excited direct- 
current machines. As will be taken up more fully 
later on, each of these three types is particularly 


Fig. 120. — Electrical Circuits—Series Machine. 

adapted for certain applications and should be used 
for these classes of service in preference to the other 
two types. We will learn later why this is so and 
what these applications are. 

Oi/fSJdeCtrqt/if 


Fig. 121. — Electrical Circuits — Compound Machine. 

While there are Homopolar machines on the 
market they are not being used very extensively up 
to the present time. In ordinary cases such as we 
have been considering, the machine must have one 
north to correspond with each south pole for correct 






110 



























Methods of Excitation 


operation. Thus in a bipolar machine there must 
be one north and one south pole and in a four-pole 
machine there must be two north and two south 
poles, and so on. If either of the windings on the two 
field poles in Figs. 122 or 123 should be reversed, 



Fig. 122. — Direction of Field Windings on 
Old-Style Bipolar Dynamos. 


changing the polarity, and producing two north or 
two south poles as the case might be, the machine 
would be rendered inoperative. The direction in 
which the windings are placed on field poles is very 



Fig. 123. —Direction of Field Windings, 
Modern Bipolar Dynamos. 


important, and Figs. 122, 123 and 124 will make 
this point plainer. 

Some of the large manufacturers of electrical 
machinery wind the field coils on forms into a spool¬ 
like shape, and after this is completed the winding 
is covered with tape or fibre and cord wrapping, for 
the purpose of insulating and protecting it. After 


ill 






















/ 


Electricity and Electrical Apparatus 

being dipped in asphaltum or some insulating varnish 
and allowed to dry, the coils are ready to be slipped 
on to the pole pieces, and in some machines, such as 
shown in Fig. 114, they are held in place by the pole 
tip or shoe. 

As we have seen, the magnetizing power of a 
coil is dependent on and proportional to the ampere- 
turns, which are the product of the turns in the coil 
by the current flowing in amperes. Thus with two 
amperes flowing in a coil of 100 turns there are 200 
ampere turns, which are sometimes known as the 
Magnetomotive-Force as they occupy the same 



Fig. 124. — Direction of Field Windings, 
Multipolar Dynamos. 


relative position in Electro-Magnetism as Electro¬ 
motive-Force or voltage does with electric currents. 

We speak of the resistance of an electric circuit 
as that property or force tending to impede or stop 
the passage of current and which must be overcome 
as current is made to flow; likewise, as we produce 
magnetic lines and cause them to circulate or flow, 
the magnetic circuit has a similar property called 
Reluctance, depending on: 

1. The Cross Section. Naturally the larger 
the circuit the easier it will be for the lines to pass, 
or vice versa as the cross-sectional area of the circuit 
is made smaller, it will be correspondingly harder 
for the lines to circulate. 

2. The Length. As a circuit is lengthened 
more Magnetomotive-Force is required to maintain 


112 





Methods of Excitation 


a certain number of lines, or as it is shortened the 
reluctance is decreased. 

3. The Material. Iron conducts lines very 
much easier than air. Also steel, and to a lesser 
extent nickel and other metals, have higher magnetic 
conductivities than air. This property of substances 
for conveying lines of force is known as their per¬ 
meability, or as it might well be expressed, their 
willingness to be permeated by magnetism. As a 
base, air is said to have a permeability of one. The 
permeability of iron and steel of course varies with 
the different grades, in some cases being even as 
much as two or three thousand times as great as air, 
which means that lines of force passing through air 
meet with that much more resistance to their pass¬ 
age. The formula for the reluctance of any mag¬ 
netic circuit is as follows: 


where 


Rel. = jp 


A = Cross-sectional area of the circuit in sq. in. 
P = Permeability of material. 

I = Length of circuit in inches. 

Pel. = Reluctance. 


The magnetizing power of a coil is proportional 
to the ampere-turns — the product of the turns in 
the coil by the amperes flowing: The actual num¬ 
ber of lines produced in any magnetic circuit equals 
the magnetomotive force divided by the reluctance; 
just as in Ohm’s Law, for electric circuits, the cur¬ 
rent in amperes is equal to the volts divided by the 
resistance in ohms. The formula for calculating 
the number of lines of force produced by any coil is: 


Number of lines = 


Magnetomotive Force 
Reluctance 


113 



Electricity and Electrical Apparatus 
3.192 XN Xl 3A92NIA P 

- ~T - = - 1 - 

AP 

where 

qp = Number of lines. 

N = Number of turns in the coil. 

I = Number of amperes flowing in the coil. 

A = Cross-sectional area of the magnetic circuit. 

P = Permeability of the material. 

I = Average length of magnetic circuit in inches. 

In any dynamo, if we know the material and 
dimensions of the magnetic circuit, and also the 
turns and the current in the field winding, we can 
calculate the actual number of lines of force in the 
field. It is very important that the material used 
and the shape of the armature core, pole pieces and 
frame be given careful consideration in any machine. 
These should be so designed as to provide a path of 
low reluctance for the lines of force. This is the 
reason why iron and steel are so widely used. While 
the air gaps between armature and fields in ordinary 
machines are but a fraction of an inch it requires 
much more energy to drive the lines of force across 
this small air space than around all of the remain¬ 
ing part of the magnetic circuit in the iron yoke, 
pole pieces and core. Joints in the magnetic cir¬ 
cuit when two pieces of iron come together will 
increase the reluctance very appreciably unless made 
very carefully. 

If the armature conductors in any machine are 
cutting only half of the lines emanating from the 
pole pieces, it is evident that the voltage generated 
and the efficiency of the machine are only half as 
great as they would be, were all the lines being cut. 
In other words, it should be our aim to have as far 
as practicable, every line pass through the arma¬ 
ture. 


114 




Methods of Excitation 


This question of magnetic leakage is quite an 
important one. It will be seen on reference to the 
illustrations that the leakage is less in Figs. 125, 127 




Fig. 126. 



Fig. 127. 



Fig. 128. 



Fig. 129. 



Forms of Pole Tips. 


and 129 than in Figs. 126, 128 and 130, the shape 
of the pole tips or shoes having a great deal to do 
with this factor in determining the efficiency of a 
machine. 










CHAPTER XIV 


VOLTAGE 

Formulas—Rheostats — Effect of Resistance in Series 
with the Field Windings on the Voltage Being 
Maintained or Generated by a Dynamo — Effect 
of Resistance in Series with Armature. 

With each revolution of the armature, shown 
in Fig. 131, every conductor will cut across the lines 
of force from both pole pieces, causing a number of 
interlinkages equal to the lines from one pole multi¬ 
plied by the number of poles, if leakage is neglected. 
The interlinkages times the revolutions per second 



gives the rate per second at which each conductor 
is cutting lines, and the quotient obtained by divid¬ 
ing this quantity by 100,000,000 is the volts being 
induced in each conductor. If we multiply the 
volts pei‘ conductor by the number of conductors, 
we get the total voltage induced in all the armature 
winding, and by dividing this total by the number 
of circuits or groups of conductors in parallel, we 
obtain the pressure at the brushes. The following 


116 



Voltage 


formula can be used to calculate the voltage gener¬ 
ated by any direct-current machine: 


where 


N XPX qo X R.P.S. 
C X 100,000,000 


N = Number of armature conductors. 

P = Number of poles. 

cp = Flux or number of lines per pole. 

R.P.S. = Revolutions per second. 

C = Number of parallel circuits in armature 
winding. 


In expressing this formula, 10 8 is often used in 
place of 100,000,000 (see foot-note) and for R.P.S. 
R P M 

is substituted ‘ . With these changes the 

60 

formula becomes: 


Volts = 


NXPX X R.P.M. 
C X 10 8 X 60 


The voltage of an operating dynamo may be 
varied by a change in the speed or the field strength. 
In ordinary cases, where engines, turbines or water¬ 
wheels are employed to drive dynamos, the. speed 
is practically fixed. The easiest and most con¬ 
venient method for obtaining different voltages is 
by field regulation. This is accomplished by in¬ 
serting resistance in series with the field windings. 
As resistance is put in, the current in the field circuit 
is decreased, causing a corresponding decrease in 
the field strength and falling off in the voltage, or 

10 8 (read “ 10 to the eighth power”) means 10 multi¬ 
plied by itself 7 times, or 10 X 10 X 10 X 10 X 10 X 10 X 
10 X 10. Also 10~ 8 (read “ 10 to the minus eight power ”) 
would mean 1 divided by 10 8 ^ or 

10 X 10 X 10 X 10 X 10 X 10 X 10 X 10 


117 






Electricity and Electrical Apparatus 

on the other hand, the voltage can be increased by 
cutting out resistance, allowing more field current 
to flow with a corresponding increase in field strength. 
The adjustable resistances used for such purposes 
as this are called rheostats. A common form is 
shown in Fig. 132. By turning the handle or knob 
the resistance is increased or decreased, depending 
on the direction in which the arm is rotated over 
the buttons. 

In building rheostats for various uses, different 
types of resistances are employed. In one form, 





Fig. 132. — Field Rheostats, General Electric Co. 

commonly used to-day, the resistance consists merely 
of coiled springs of German silver or other high- 
resistance wire. For handling large currents, the 
well known grid rheostats are used, the resistance 
element being cast iron. 

In these rheostats the covers are perforated or 
the supports made so as to allow a free circulation 
of air, for cooling. There is quite a little energy 
expended in any high-resistance wire or metal, as 
current flows, and consequently, to prevent ex¬ 
cessive heating, a very important point in rheostat 
design and building is the means provided for cool¬ 
ing. Also, in installation, their location on switch- 


118 








Voltage 


board or elsewhere, should be in a position removed 
from any inflammable material and such that the 
heat radiated can do no harm. 

In the so-called “ pressed card” type of rheostat, 
the resistance wire is wound around an asbestos tube 



Fig. 133. — General Electric Grid Rheostat. 

or cardboard cylinder, which is pressed flat and 
placed in rheostat. 

For some purposes it is customary to employ 
water rheostats, consisting essentially of a box or 



Fig. 134. — Connection Diagram for Field Rheostat. 

tank filled with water. Often the tank is of cast 
iron, or else the inside is lined with some metallic 
substance, as sheet iron. One side of the circuit is 
attached to this metal lining or box, the other side 
being connected to a movable cast-iron plate, which 


119 
























Electricity and Electrical Apparatus 

can be lowered into the water to any desired depth. 
The resistance offered to the flow of current through 
this rheostat depends on the amount of the movable 
plate surface immersed, and on the purity of the 
water. If salt is added, or other impurities present 
in the water, its resistance will be lessened. 

Fig. 134 is a connection diagram of a rheostat 
in the field circuit of a shunt machine for voltage 
regulation. 

The voltage of a dynamo might be lowered by 
inserting resistance in series with the armature, as 
shown in Fig. 135, but the total line current would 
be carried, and the very large rheostat necessary to 
handle this current would be undesirable on account 
of bulkiness and the power consumed. 



Fig. 135. — Resistance in Series with Armature. 

Up to the present time, the self-excited machines 
considered have been already in operation. At the 
close of a day or night’s run, as one of these is being 
shut down, its voltage will “fall off” with the dying 
down of the speed, becoming less and less until it 
is zero as the machine stops altogether. The ques¬ 
tion now arises, when we wish to start up the next 
time: How will voltage be produced, without using 
a separate exciter? 

After the machine is shut down and the field 
current turned off, the pole pieces do not lose quite 
all of their magnetism. These magnetic lines, still 
residing in the pole pieces, are commonly called the 
residual magnetism, and although comparatively few 
in number they ordinarily are sufficient for the 


120 





Voltage 


purpose of building up the voltage during the sub¬ 
sequent starting of the machine. As the arma¬ 
ture begins to turn in this residual field, a small volt¬ 
age is induced in the conductors. This causes, in 
turn, a small current to flow through the field wind¬ 
ings, augmenting the residual and inducing corres¬ 
pondingly more voltage in the armature conductors. 
In this way the voltage is brought to normal. 

A necessary precaution in this proceeding, is 
to be sure the field coils are connected properly 
with respect to armature. If, after a machine is 
shut down, someone reversed the field wires to the 
armature, the small initial voltage induced by the 
residual would send a current through the field 
windings in such a direction as to buck and neutral¬ 
ize the residual, leaving the - pole pieces with no 
magnetism whatever. In this state it would be 
necessary to use a separate exciter or some batteries 
to set up a few initial lines of force. 


121 


CHAPTER XV 


ELECTRIC MOTORS 
Cause for Rotation. 

The dictionary defines Motor as a machine 
which does work. Under this classification an electric 
motor would be a machine doing work and operated 
by electricity. Many of the applications of electric 
motors are familiar to all, such as their use on street 
cars, elevators, in machine shops, operating venti¬ 
lating fans, sewing machines, ice cream freezers, 
milk separators and countless other uses. There 



are several kinds of electric motors. Each type, 
owing to its distinguishing characteristics, is espe¬ 
cially adapted for certain purposes and should be 
used where these particular features can be employ¬ 
ed to the best advantage. 

Any ordinary dynamo or generator capable of 
producing current can be operated as a motor if 
connected to a source of electric current. The force 
causing the motor armature to rotate is due to the 


122 












Electric Motors 


reaction upon the magnetic field by the currents in 
the armature conductors. Referring to Figs. 137, 
138 and 140, the conductor carrying current towards 
the observer or upwards through the paper sets up 
circular lines of force in a counter-clockwise direction, 
and if placed in a magnetic field the whirls produced 
by the current will combine with the field from the 
magnet, distorting and causing the lines of force to 



Fig. 137.— Conductor in Magnetic Field, 
No Current Flowing. 


assume curved shapes as shown. The tendency of 
lines of force to follow the shortest, or path of least 
resistance, causes them to have a contracting action 
like stretched rubber bands, and there will be a 



Fig. 138.—Magnetic Field Fig. 139.—Magnetic Field 
around Conductor with around Conductor with 

Current Flowing Toward Current Flowing From 

Observer. Observer. 


resulting force tending to push the conductor in the 
direction indicated by the arrow. 

If the direction of a current in a conductor is 
downwards or away from the reader, the resulting 
whirls will be in a clockwise direction, and if placed 


123 







Electricity and Electrical Apparatus 


in a magnetic field the force resulting will tend to 
cause the movement of the wire as shown in Fig. 141. 

This force is proportional to the current in the 
conductor and the magnetic density, in lines per 
square inch. 



Conductor Carrying Current in Magnetic Field, Show¬ 
ing Distortion of Field and Direction of Result¬ 
ing Push on the Conductor. 


If a coil of a single turn is placed in a magnetic 
field as shown in Fig. 142 there will be a tendency 
for it to rotate in a clockwise direction if current 
flows as indicated. If the]direction of flow of current 




Fig. 143. 


Current-Carrying Loop or Coil in a Magnetic Field, 
Showing Direction of the Tendency to Rotate. 


is reversed in this coil, the polarity of the field re¬ 
maining the same meanwhile, the direction of the 
rotation is reversed. 

In an ordinary machine, if the armature is con¬ 
nected to a circuit and current allowed to flow, all 


124 





































Electric Motors 


of the conductors under the north pole will carry 
currents flowing in the same direction, and all of the 
currents under each south pole will form a broad 
sheet of parallel currents, but flowing in an opposite 
direction. If the field coils are magnetized in this 
machine while current is flowing through the arma¬ 
ture, rotation will be caused. 

The torque, that is, the turning effort or force 
tending to produce rotation, in any motor is propor¬ 
tional to the armature current, the strength of mag¬ 
netic field and the number of armature conductors. 

The direction of rotation in any direct-current 
motor will be changed if the current flowing through 
the armature is reversed, provided that the polarity 
of the fields remain the same. If both the field 
magnetism and direction of armature current are 
reversed, the motor will continue to operate in the 
same direction. 


125 


CHAPTER XVI 


ELECTRIC MOTORS ( Continued) 


Armature Drop — Back Electro-Motive Force — Dis¬ 
cussion of These Two Quantities and of the Fact 
that Their Sum Equals Line Voltage — Starting. 

Suppose the resistance of a shunt motor arma¬ 
ture is known to be .15 of an ohm. This figure does 
not represent the whole winding in series, but the 
effective resistance offered to the flow of current 
through the armature from the positive to the nega¬ 
tive brush, or in other words, the resulting resistance 
of the parallel circuits in the winding. If the motor 
was operating free, with the armature leads connected 
directly across a 115-volt line, it would at first seem, 
from Ohm’s Law, that the armature current would 
be 767 amperes, as follows: 


Current = 


Volts 

Resistance 


115 

.15 


= 767. 


But as a matter of fact, in actual tests on a 5 H.P. 
motor with an armature of the above resistance, the 
free current is approximately 5 amperes. Under 
ordinary conditions .75 volts would cause 5 amperes 
to flow through a resistance of .15 ohms, while here 
we have 115 volts, and can only account, apparently, 
for .75 volts. The question is: What has become of 
the other 114.25 volts? 

As a motor armature rotates there is an E.M.F. 
induced in each of the conductors; this is necessarily 
so, as they are cutting lines of force from the field 


126 



Electric Motors 


magnets. These voltages, as can be seen from Fig. 
136, are opposite in direction to the line voltage and 
will buck against and neutralize a certain portion 
of it, depending on the strength of the magnetic 
field of the motor and on its speed of rotation. It 
will be noted that the voltage in the generator is in 
the same direction as the current, while the voltage 
induced in the armature conductors of the motor 
opposes the flow of armature current. Thus if a 
motor is running at certain speed, such as to generate 
114.25 volts, this counter electro-motive force will 
neutralize an equal amount of the line voltage and 
leave only .75 volts effective for causing current to 
flow. The sum of the armature-resistance drop in 
volts and the back E.M.F. of a motor equals the line 
voltage, which can be expressed as a formula: 

Line Volts = Armature Resist. Drop + Back E.M.F. 

= R a C a +V m . 

Where R a = Armature effective resistance. 

C a = Armature current (total). 

V m = Back E.M.F. or voltage generated by 
motor. 

To drive a machine, such as a lathe, the torque 
or turning effort required will depend on the work 
being done. A motor-driven lathe operating free 
requires small torque. As the tool commences to 
cut, thereby putting on load, the torque is insufficient 
and the motor speed will be lowered. As the motor 
slows down, the counter E.M.F. is decreased and 
more current will consequently flow through the 
armature, increasing the torque and enabling the 
motor to pull a heavier load than before. On the 
other hand, if the load on a motor is lightened, it 
will speed up, increasing the back E.M.F. With 
the consequent cutting down of the armature current 
the torque decreases until equilibrium is restored 
and the speed becomes constant. 


127 


Electricity and Electrical Apparatus 

Thus a motor is, to a certain extent, automatic 
in its action — more current flowing as the load 
increases and smaller currents passing through the 
machine under lighter loads. In steam engines a 
governor is required to regulate and admit the proper 
amount of steam to the cylinder with each stroke 
of the piston. This phenomena is taken up more 
fully later on and its effect upon the operation of 
shunt, series and compound-wound motors ex¬ 
plained. 

The free speed of the motor above mentioned 
was 1,500 revolutions per minute, and when de¬ 
livering 5 H.P. the speed was 1,430 revolutions per 
minute, while the line current increased from 5 to 
40 amperes. Under the loaded conditions the 
armature resistance drop would be 40 X .15 = 6 
volts, and the counter E.M.F. would then necessarily 
be decreased to 109 volts. Also it will be noted 
that the speed decreased in proportion: 

1500 _ 115 
1430 109‘ . 

In the case of series motors, however, this speed 
variation due to changes in load, gives rise to 
dangerous effects. If the load is quickly taken off 
a series motor in operation it will at once speed up. 
This higher speed raises the back E.M.F. and cuts 
down the current through armature and fields. 
This decrease in current weakens the field, and the 
motor speed continues to rise, in an endeavor to 
maintain the proper back E.M.F. This produces 
the well known tendency of series motors to “run 
away” and is the reason why they should always 
be geared to load instead of belting or other means 
more or less likely to give away and remove the load 
from motor. 

In any motor, if the strength of the field is 
altered the speed will be affected. With a decrease 


128 



Electric Motors 


in field, the back E.M.F. is lessened temporarily 
allowing more current to flow through the armature 
circuit, and in ordinary cases, where the load is not 
excessive, causing the speed to rise. If we con¬ 
tinue to lessen the field current the speed will rise 
higher and higher, and finally as the field current 
is brought to zero, the speed of the motor will have 
to become infinite in order to generate a sufficient 
back E.M.F. For this reason it is very dangerous 
and disastrous if the field circuit is broken while a 
shunt motor is in operation, the excessive currents 
burning the commutator surface and usually flash¬ 
ing over from brush to brush. In a series motor, 
opening the field circuit will of course shut off cur¬ 
rent from the armature. 

On the other hand, any increase or strengthen¬ 
ing of the field in a motor will cause it to slow down. 
This is due to the fact that as the field is made 
stronger the counter E.M.F. becomes greater, tem¬ 
porarily cutting down the armature current to a 
smaller value and lowering the torque or turning 
effort. This will cause the motor to slow down in 
the course of a short period of time. However, in 
the special case where the armature is large and 
heavy, its momentum might be sufficient to momen¬ 
tarily generate a counter E.M.F. equal to the line 
voltage, or possibly even greater. If the counter 
E.M.F. is the greater it will actually overcome the 
line voltage and force or pump current back into 
the line, the motor acting temporarily as a generator. 
Motor control and speed regulation is taken up more 
fully in the following chapters. 

If a shunt motor at rest was thrown directly 
across the line there would be no counter E.M.F., 
hence, an enormous current would flow through 
the armature windings, causing severe heating and 
flashing at the brushes, and in some cases arcing from 
stud to stud, which burns the commutator and is 
dangerous to attendants. To prevent this, a start- 


129 


Electricity and Electrical Apparatus 

ing box is used (Fig. 144), resistance being connected 
in series with the armature. 

In starting a shunt motor, it is of vital im¬ 
portance to first have the field connected across the 



Fig. 144. — D.C. Motor-Starting Box, Arm in 
Off Position. General Electric Co 

source of current with about normal excitation. 
Then, the motor is started and gradually brought 
up to speed by slowly pulling the rheostat arm across 



Fig. 145. — D.C. Motor-Starting Box. Cover Removed, 
Showing Resistance Units. General Electric Co. 

the buttons, cutting out, step by step, the resistance 
in series with the armature (See Fig. 146). 

With a series motor, a similar starting box is 
employed, the resistance being in series with both 


130 





Electric Motors 



Fig. 146. — Connection Diagram, Starting Box for 
Shunt Motor. 



Fig. 147. — Connection Diagram, Starting Box for 
Series Motor. 

armature and fields. A series motor is much less 
liable to flash over than a shunt-wound in starting, 
as the field is immediately strengthened whenever 
the current tends to become excessive. This raises 


131 









































Electricity and Electrical Apparatus 

the back E.M.F. and prevents further increases in 
current and exerts a tendency to hold the current 
down to normal values. As the field of a shunt 
motor is approximately constant, the excitation 
being independent of armature current, there would 
not be as great a factor of safety in starting. 

A second way in which the speed of motors may 
be varied is by inserting a rheostat in series with 
the armature. As the resistance is put in series 
with the armature, the current flows first through 
the rheostat and then through the armature. Due 
to the drop across the rheostat, the effective voltage 
on the armature is less, consequently the motor will 
slow down. This method cannot be used to raise 
the speed, and is rarely utilized to lower the speed, 
as considerable power is consumed. 


132 


CHAPTER XVII 


ARMATURE REACTION 

In a two-pole motor or generator the lines of 
magnetic force set up by main field coils will follow 
a path as shown in Fig. 148, providing there is no 
current passing through the armature conductors. 
The direction of flow of these lines of force of course 



Fig. 148. — Path of Flux Produced by Field Windings, 
Acting Alone. 

being determined by direction of current in field 
coils. On the other hand, if we omit the main field 
flux altogether and now consider the armature con¬ 
ductors carrying current as shown in Fig. 149, we 
find the iron core of the armature will act like a 


133 






















Electricity and Electrical Apparatus . 

powerful magnet, as the armature winding sets up 
two distinct poles, one N, the other S. 

In an armature built for multipolar machines 
there are as many magnetic poles on the armature 
as there are pole pieces in the frame of the machine. 
These armature magnetic poles are midway between 
main poles of frame. In a bipolar machine the 
separate fields set up by the armature winding and 
main field are at right angles to each other. The 



Fig. 149. — Path of Flux Produced by Armature 
Current, Acting Alone. 

combination of these two fields or resultant flux 
is shown in Fig. 150, the rotation of armature being 
clockwise. The direction of this resultant flux can 
be more clearly understood by noting that at pole 
tips A and D the main and armature fields are in 
the same direction and therefore augment each other, 
while at pole tips B and C they flow in opposite 
directions and oppose. 

The amount of deflection of main flux depends 
on the relative strength of the armature and main 


134 



Armature Reaction 



Fro. 150. — Distorted Field, Resulting from the Com¬ 
bination of Field and Armature Flux, in a 
Generator. 



Diagrams Showing that Amount of Distortion Depends 
on Relative Strength of the Two Fields, and 
that Weak Armature and Strong Field will 
Cause Little Distortion as Aramture Current 
Varies. 


135 




















Electricity and Electrical Apparatus 

fields. If the main field is strong and armature 
field weak, any change of the amount of current in 
armature conductors and consequent change of 
armature fields causes little deflection of main flux. 
On the other hand, in a weak main field and strong 
armature any change in armature current brings 
about marked change in direction of resultant flux. 
These facts may be clearly shown in Figures 151 and 
152. 



In Fig. 151, let OM represent direction and 
magnitude of main field, while OA represents di¬ 
rection and magnitude of armature field, and OR 
direction of resultant lines of force. Any change 
in OA affects direction of OR very little. But in 
Fig. 152 any change in OA affects direction of OR 
very materially. 

This description is based on a generator. In 
a motor with same polarity of main field and direction 
of rotation the armature current would be reversed. 


.136 










Armature Reaction 


This change of direction in armature current reverses 
the lines of force in the armature so that the re¬ 
sultant flux instead of being like Fig. 150 will be 
up and to the right as shown in Fig. 153. 

Armature Reaction is the reacting effect of the 
current in armature conductors upon the main field, 
causing the distortion as explained above. 


137 


CHAPTER XVI11 


ELEMENTARY IDEAS OF COMMUTATION 

The direction of current in the revolving arma¬ 
ture conductors reverses as they pass out from one 
pole and under the next. Due to this change, it is 
necessary, in order to obtain a continuous current, 
for leads from certain places in the armature wind¬ 



ing to be connected with a segment ring or com¬ 
mutator. Current is conducted to or from the com¬ 
mutator, as the case may be, by carbon brushes or 
blocks of carbon sandpapered to fit the curving 
surface of the commutator. These brushes, except 
when resting on one segment, of necessity short- 
circuit part of the winding, and as brushes are 


138 
















Elementary Ideas of Commutation 

generally of such width as to cover at least two 
segments, this short circuit always exists. Fig. 154 
represents a two-pole generator, armature revolving 
in clockwise direction. Inspection shows that coils 
1-8 and 9-16 are short-circuited by the brushes. As 
the armature revolves the different coils are short- 
circuited by brushes, but always the coils having 
same relation to brushes as coils 1-8 and 9-16 in 
Fig. 154. 



Assuming no armature current and therefore 
no armature reaction, the main field then flows 
direct from N to S pole with no distortion. As the 
conductors 1-8 and 9-16 are moving practically 
parallel to the lines of force, the voltage generated 
in short-circuited {coils is very small. The high- 
resistance carbon brushes keep down what current 
would tend to flow in these coils. 

In Fig. 154, the voltages being generated in con¬ 
ductors 14 and 7 are such as to cause current to flow 


139 












Electricity and Electrical Apparatus 

in directions indicated. Current flows from external 
circuit into brush and thence into segment 5, and 
from segment 5 to conductor 14. Tracing current 
from 14 to 7, to segment 4, and then to conductor 
12, to 5, to segment 3, and .then to conductor 10, to 3, 
and then to segment No. 2, the current leaves com¬ 
mutator for external circuit by means of positive 
brush, the voltages generated in conductors 14, 7, 



12, 5, 10 and 3 being added together. Another 
circuit from the negative brush exists. This is: 11-2 
to 13-4 to 15-6 and from segment 1 to positive brush. 
The main current divides through the two circuits, 
one-half being carried by each circuit. The vol¬ 
tages being generated in conductors 2, 3, 4, 5, 6, and 
7 cause current to flow toward observer at instant 
shown in Fig. 154, while in conductors 10, 11, 12, 

13, 14 and 15 the direction is away from observer. 
Fig. 155 shows armature advanced one-eighth 

revolution and conductors 14 and 7 are now in 


140 
















Elementary Ideas op Commutation 

neutral zone, or moving parallel to lines of force, and 
although short-circuited, no dangerous current will 
flow due to the motion parallel with the lines of 
force and the high resistance the carbon brushes 
introduce in the circuit. 

In Fig. 156, an eighth of a revolution later, con¬ 
ductors 14 and 7 have begun to cut the lines of force 
again and current flows as illustrated. This will 
continue for another three-eighths revolution, until 
the coil gets to a position occupied by coil 4-13, in 
Fig. 156, when it is commutated once more. 

It should be noted that, due to the method of 
connection, the voltages generated in the conductors 
as they sweep across pole faces are cumulative, and 
that while conductors are short-circuited by brushes 
they are moving so as to cut few lines of force and 
generate little or no voltage. 

This discussion is based on no armature reaction, 
or at a condition of no load, which means little arma¬ 
ture current for a motor and none at all for a genera¬ 
tor. We should know, however, that when loaded, 
machines have armature reaction. A machine 
with brushes set to commutate coils midway between 
pole tips, as in Figs. 154,155 and 156, would probably 
spark badly on heavy loads, due. to currents induced 
in short-circuited coils by the distorted field due to 
armature reaction. Thus for the best commutation 
we must either shift the brushes, or counteract the 
armature reaction. 


141 


CHAPTER XIX 


BRUSHES 

Shifting and Setting of Brushes — Reasons for Shift¬ 
ing Brushes Forward from Neutral in a Genera¬ 
tor, and Backward from Neutral in a Motor — 
Sparking — Inter poles. 

The distortion of the field flux due to armature 
reaction is proportional to the current in the arma¬ 
ture conductors. The stronger the current in the 
ar-mature the greater the distortion or reaction on 
the field. Thus when a motor is running at light 
load or free, the armature current will be small and 
the distortion of the field will be inappreciable, while 
on heavy loads with the armature current increased 
to a large value, this distortion is sometimes very 
pronounced, causing the machine to spark at the 
brushes. 

The causes for this sparking are many and varied 
but a few of the more important factors essential 
to good commutation are readily understood and 
will now be taken up. During the period of time 
a coil is short-circuited by the brush, the current 
must be brought to zero and then set up again in a 
direction opposite to that in which it was flowing 
before undergoing commutation. When a heavy 
load is put on a machine the distortion of the main 
field is so great as to cause a number of the lines of 
force to thread or interlink with the short-circuited 
coil. The cutting of these lines of force induces a 
voltage, and heavy currents will be set up, causing 
sparking and dangerous heating of brushes and arma¬ 
ture. The sparking is caused by the segments con- 


142 


Brushes 


nected to short-circuited coils passing out from 
under the brush, thus breaking the short circuit. 
To prevent this cause of sparking the brushes should 
be shifted to a position where short-circuited coils 
will cut as little as possible of the active flux. In a 
generator this position is found by moving brushes 
forward from mechanical neutral around com¬ 
mutator, that is, with direction of rotation; for a 
motor, brushes should be moved backward. This 



setting is called giving a generator “forward lead” 
or a motor “backward lead.” To correctly set the 
brushes of a generator or motor, first trace out lead 
from the coil midway between pole tips to com¬ 
mutator segment. Then from this segment give 
brushes from 1 to 3 segments lead forward for gene¬ 
rator and backward for motor, the amount of lead 
depending on best observed commutation results. 


143 

























Electricity and Electrical Apparatus 

To more clearly show the necessity of lead in 
brush setting refer to Figs. 157,158 and 159. Fig. 
157 shows motor field when running at no load, Fig. 
158, the full load armature field alone, and Fig. 159 
the resultant field and distortion. In order to bring 
short-circuited conductors to a position where the 
flux cut during commutation will be small, it is 
necessary to give brushes a backward lead, sufficient 



to bring conductors on line AO A. And in practice 
it is necessary to give brushes even a slightly greater 
lead than this; that is, set brushes so that com¬ 
mutated coils actually cut a small amount of flux 
from the weakened tip of the next adjacent pole, 
thereby inducing in short-circuited coils a small 
voltage which in turn tends to set up a current in 
conductor in opposite direction to the flow in coil 
before being short-circuited, thus not only bringing 


144 














Brushes 


the current to zero but actually starting a current 
in the reverse direction. 

This extra shift of brushes is necessary because 
an armature is an inductive circuit. Any coil wound 
around an iron core is an inductive circuit and pos¬ 
sesses the peculiar power of maintaining the flow 
of an electric current for a short time after the supply 
voltage is shut off. When an armature coil is 
short-circuited it is of course isolated from an ex¬ 



ternal source of current. The current flowing in 
these coils when short-circuited does not immedi¬ 
ately die but consumes an appreciable time, and 
while this time is short and but a small fraction of a 
second, it is in many cases sufficiently great for the 
coil still carrying some of this inductive current to 
pass through the commutating zone and enter the 
circuit again, when it will begin to receive current 
from the line in the opposite direction. 


145 














Electricity and Electrical Apparatus 

The line current and the inductive current are 
flowing in opposite directions. As the coil emerges 
from the short-circuit region, still carrying inductive 
current, there is a resulting opposition which ac¬ 
centuates the sparking. It is, therefore, necessary 
to place the short-circuited conductors in a position 
of active flux so as to generate sufficient voltage 
not only to oppose the inductive current but to 



Fig. 160 . — Commutating Zone in a Motor. 


completely reverse the direction of the current in 
the conductor as it passes from one side of the com¬ 
mutating zone to the other. 

In Fig. 160 consider conductor C revolving as 
shown. When it reaches D it is about to enter the 
commutating zone ZOZ or that part of its revolution 
when it is short-circuited by one of the brushes. As 
soon as it has reached the position E the coil must 
take current in the opposite direction. If now the 


146 
















Brushes 


inductive current left in a conductor while it passes 
through the short-circuit zone is not neutralized 
sparking will occur as inductive current opposes 
line current at E. If the brush position is changed 
to give a new commutating zone Z'OZ 1 , thus bring¬ 
ing the short-circuited conductor into active flux, 
a voltage will be generated in the conductor during 
its passage through commutating zone of such 
strength as to not only neutralize the inductive 



Fig. 161 . — Demagnetizing Conductors in a Motor 
Armature. 

current but actually set up a small current in the 
opposite direction so that conductor will take line 
current without opposition. This explanation ap¬ 
plies equally well to the generator. The only differ¬ 
ence is that a forward lead is required instead of a 
backward. 

If a machine sparks more at no load than at 
full load, it indicates too much lead and brushes 
should be shifted to overcome this. 


147 







Electricity and Electrical Apparatus 

The backward lead on a motor causes the cur¬ 
rents in a certain number of the conductors on the 
armature to set up a field in opposition to the main 
field thereby reducing the main flux and for this 
reason causing the motor to increase in speed. In 
a generator this same condition exists with a for¬ 
ward lead causing the field and consequently the 
voltage to be reduced. Fig. 161 shows the con¬ 
ductors on the armature of a motor. ZOZ shows 



Fig. 162 . — Demagnetizing Conductors in a Generator 
Armature. 

commutating zone. Conductors a, a, a, etc., and 
b, b, b, etc., set up a flux in opposition to main field 
thereby reducing its value and speeding up motor. 
Fig. 162 shows same condition for a generator. These 
turns a, a, a, etc., and b, b, b, etc., are called “back” 
or “demagnetizing ampere-turns” and their number 
depends on amount of lead. An armature with no 
lead has no back ampere-turns. 

Brushes should of course be made to fit com¬ 
mutator surface by sandpapering. Never use emery 


148 








Brushes 


cloth as emery is a conductor and particles remain¬ 
ing between commutator segments cause serious 
trouble by short-circuiting coils connected to the 
segments. Brushes are made of carbon, as has been 
explained, in order to introduce a high resistance 
between segments; and for good commutation it is 
necessary that these brushes should have a perfect 
fit on the surface of the commutator. The surface 
of the commutator is often rough, due to the mica 
insulation between the segments, which sometimes 
works loose and causes an uneven surface. This 
can be corrected by a careful application of sand¬ 
paper when commutator is revolving. It is always 
advisable to remove the brushes from the commuta¬ 
tor during this process. The surface of the com¬ 
mutator may be so rough as to necessitate the 
removal of the armature to a lathe where the com¬ 
mutator can be tightened and its surface turned to 
a new finish. In some cases, particularly with 
large machines, it is not practical to remove the 
armature, and by using some standard commutator 
truing device a new surface can be given commutator 
while rotating on its own bearings. 

Current density in carbon brushes should not 
be over 25 amperes per square inch. The density 
on small motors of \ H.P. and below is often very 
much less than this value as on these machines the 
governing factor is to have the brush large enough 
for ample mechanical strength and size. Copper 
brushes are only used on special 5 to 10-volt genera¬ 
tors and watt-meters. These low-voltage generators 
are used for electroplating or in the manufacture of 
permanent magnets where large currents are re¬ 
quired. 

Commutating Pole Construction 

In any ordinary machine not fitted with com¬ 
mutating poles it has been shown that a backward 
lead for motor and forward lead for generator is re- 


149 


Electricity and Electrical Apparatus 

quired in order to bring commutated coils into 
sufficient live flux to neutralize the inductive cur¬ 
rent left in the coils when they enter the commutat¬ 
ing zone, and also to establish the small current in 
the opposite direction in these coils. If now this 
flux could be introduced midway between the pole 
tips no lead would be necessary for either the motor 
or generator, as the necessary commutating field 
would be found at this position rather than at a 
position under the main poles which necessitates a 



Fig. 163 . — Inter-Pole Direct-Current Motor, Electro- 
Dynamic Co. 

lead of the brushes. Where commutating poles are 
used brushes would then be set to short-circuit 
coils midway between pole tips. Figs. 163 and 164 
show a commutating pole motor. The small poles 
are called commutating or interpoles and are placed 
midway between the main poles. The field coils 
on these small poles are energized by the armature 
current and are so connected as to magnetically 
oppose the armature flux and so designed as to not 
only neutralize the armature field under the pole 
face but to over-balance it, thus sending into the 


150 


Brushes 


armature the slight amount of flux necessary to 
neutralize the inductive current above referred to. 

As explained, the commutating coils are con¬ 
nected in series with the armature, therefore on light 
loads the commutating poles are weak while on heavy 
loads they are strong and at all times automatically 
regulate the number of lines of force passing through 
the short-circuited coils directly under the poles. 
In machines not fitted with commutating poles the 
theoretically correct position of setting brushes 
changes with the load, that is, the armature flux 



Fig . 164. — Internal View of an Inter-Pole Motor. 

distorts the main field more and more as the load 
increases, thereby requiring greater and greater lead 
to keep commutated coils in theoretically correct 
position. In the commutating-pole machine this 
change of lead is dispensed with as the commutating 
flux is supplied by the commutating poles. 

Machines fitted with commutating poles can 
be made to operate successfully with shorter air gap 
between armature and pole-faces and less flux than 
machines not fitted with these poles. This means 
that motors and generators so constructed can be 


151 


Electricity and Electrical Apparatus 

made lighter and cheaper for the same output. 
Motors of this construction also offer better regu¬ 
lation, that is, the change of speed from no load to 
full load is less than ordinary machines, making them 
exceptionally desirable for driving machine tools. 
Figures 165 and 166 show a two and four-pole com¬ 
mutating pole motor. 

The subject of commutating pole construction 
will be more fully taken up in Volume II of this 



Fig. 165. — Bipolar Inter-Pole Motor. Magnetic 
Flux Distribution. 

text-book.* Also an extract of an article by the 
authors in the May 5th, 1910, issue of the Electrical 
World is given below. 

It is generally known that direct-current motors 
and generators equipped with commutating poles 

* An interesting discussion of commutating poles will 
be found under Chapter I, Question 9, “Questions and 
Answers About Electrical Apparatus,” by the authors. 


152 























Brushes 


possess many advantages over similar or old-type 
machines not so equipped. To equip shunt, series 
or compound-wound, direct-current motors and 
generators with commutating poles is a compara¬ 
tively simple matter, and it is the purpose of this 
article first to show and briefly explain advantages 



Fig. 166. — Four-Pole Inter-Pole Motor. 
Flux Distribution. 


of this construction, and then to make clear the 
practical calculations and applications of these 
auxiliary poles and coils. 

The chief advantages of commutating-pole 
machines are high efficiency, cooler parts, improved 
commutation under all load conditions, and in the 
case of shunt motors better speed regulation. 


153 






















Electricity and Electrical Apparatus 

The commutating poles, while adding to the 
copper losses of the machines, so reduce the short- 
circuited currents in the armature and brushes as 
to show high efficiency. The reduction in the 
short-circuited armature current permits not only 
the armature, but the commutator as well, to operate 
at a lower temperature. This fact renders the use 
of commutating poles very valuable, because hot 
commutators tend to cause sparking due to uneven 
surface and high mica. 

The commutating poles should be set midway 
between the main poles so that their flux acts in 
• the same capacity as the fringe of flux from the 
main poles previously referred to. The field coils 
on the auxiliary poles are connected in series with 
the armature in such a manner that the commutating 
pole flux opposes the effect of the armature re¬ 
action. The flux of the poles then increases and 
decreases in strength with the armature current, 
and is, therefore, able to offset the effect of the 
armature reaction under their faces which on these 
machines is the zone of commutation, and the 
brushes are always set so as to short-circuit the coils 
under the commutating poles. The backward 
motor and forward generator lead in setting the 
brushes is avoided. Figure 165 shows a motor fitted 
with commutating poles indicating the setting of 
the brushes. 

In shunt-wound motors equipped with com¬ 
mutating poles the speed can be adjusted by shunting 
the commutating field coil current to give practi¬ 
cally a “flat” regulation. A drop in speed from 
no-load to full-load of only two per cent, is perfectly 
feasible; closer regulations are undesirable because 
a commutating-pole motor becomes unstable when 
the speed curve is too near a horizontal line. 

The commutating pole construction increases 
the speed of motors by from five per cent, to eight 
per cent, on small machines and from two per cent. 


154 


Brushes 


to five per cent, on larger sizes; it also lowers the 
voltage of generators by similar amounts. The in¬ 
crease of speed in motors and the decrease of volt¬ 
age in generators is due to a reduction of the main 
flux by the interception of the commutating pole 
flux. The lower the main flux densities, the lower 
is the percentage of change. 

Fig. 166 shows the flux at full load on a four- 
pole motor fitted with commutating poles; it should 
be noted that one-half of the main flux circuit is 
parallel to the commutating pole flux, thereby in¬ 
creasing the reluctance of the main flux path for 
that part of the circuit and consequently lowering 
the main flux and increasing the motor speed or 
lowering the generator voltage. 

The Addition of Commutating Poles to Old Machines 

It is a good practice to use as many commuta¬ 
ting poles as there are main poles. The pole-pieces 
should be made of cast-steel forgings or sheet-steel 
punchings riveted together. Cast iron should not 
be used as its capacity for carrying flux is much less 
than that of steel or wrought iron. The poles should 
be of such length as to extend from the frame, to 
which they should be bolted, to the armature, 
leaving an air-gap not less in depth than the gap 
between the main poles and the armature. 

The cross-sectional area of the pole is important. 
The thickness should be approximately three times 
the width of the armature slot. The width of the 
pole should be not less than seventy per cent, of 
the armature-core iron. The face of the pole need 
not be bored, but can be left flat. Fig. 167 illus¬ 
trates these points. 

The commutating pole coils should be wound 
with insulated wire, or bare copper wire strap with 
varnished cambric between layers. Terminals or 
ends of straps should be brought out on the ends 


155 


Electricity and Electrical Apparatus 

of the coils, not on the sides, where interference with 
main coils will result. Wire or strap should be 
wound on a suitable insulating box and then dipped 
in black japan. Black coils run cooler than white. 

The cross-section of the wire or strap depends 
on the value of the armature current. The current 
density should be from 1250 amp. to 1350 amp. 
per square inch. 

The formula for calculating the number of 
turns per coil is T = (0.6XWX£) ( PXC ), where 
W = number of conductors per armature slot, two 



Fig. 167. — Comparative Size of Commutating Pole. 

or more wires in multiple being counted as one 
conductor; S = number of slots; P = number of 
main poles; C = number of circuits in armature. 
C = 2 for all series-connected armatures and for 
multiple-connected armatures C = P. 

As an example illustrating the use of the above 
formula consider a four-pole motor having fifty- 
nine armature slots with twenty-six conductors per 
slot, the armature being series-connected. Then 
T = (0.6 X 26 X 59) (4X2) = 115.5. 


156 



Brushes 


One should always calculate to the nearest 
half turn. The armature current can be determined 
from the name-plate, which gives the value of the 
full-load current. For a shunt-wound motor, the 
armature current equals the load current less the 
field current. In a shunt-wound generator the arm¬ 
ature current equals the load current plus the field 
current. The value of the field current can be 
ascertained by means of an ammeter. 

After the connections have been made, the 
machine should be operated with a small amount of 
current; a pocket compass should be. used to de¬ 
termine if the polarity is correct. The brushes 
should then be so set as to short-circuit the con¬ 
ductors under the commutating pole faces. The 
machine should be run as a motor, if possible, at 
light load, to determine the exact setting of the 
brushes to give the same speed in either direction 
of rotation. The brush position should be care¬ 
fully marked, and the machine should subsequently 
be run with the brushes in this position. This set¬ 
ting of brushes should check very closely with the 
preliminary setting of the brushes referred to above. 
There should be connected a german silver shunt 
or water rheostat around commutating field coils, 
and the machine should be operated at full load and 
then double load if possible, the shunt being ad¬ 
justed to give the best results; final adjustments 
should be made with the machine hot. The shunt 
should be arranged for the best commutation with¬ 
out making the machine as a motor unstable. Too 
much commutating-pole field flux will cause a motor 
to “hunt” or race. During the trial setting of a 
motor an attendant should always be stationed at 
the main switch to open the circuit at the first signs 
of hunting. No trouble of this kind is experienced 
with commutating-pole generators. After the re¬ 
sistance of the shunt has been adjusted a permanent 
shunt resistor can be made up and installed in cir- 


157 


Electricity and Electrical Apparatus 

cuit in place of the temporary shunt, or the com¬ 
mutating pole field coil can be removed and turns 
taken off to give the correct field strength. The 
correct number of turns can be determined from the 
values of the necessary current in both commutating 
field coils aiid the temporary shunt when the machine 
is operating at or near full load after a run of suffi¬ 
cient length to acquire its normal temperature. The 
new number of turns T n can be calculated by means 
of the formula: T n = I^T -f- (I x + I 2 ), where h = 
current in the compensating field coils, / 2 = current 
in the shunt around these coils, and T = the num¬ 
ber of turns in one of the original coils. 

As has been mentioned above, a commutating- 
pole motor will “hunt” if incorrectly adjusted. 
Hunting on all types of commutating-pole motors 
should be carefully guarded against, because serious 
injury may result, both to the attendant and to the 
machine, from any excessive speed resulting from 
the hunting action. Hunting is caused by the 
commutating field over-powering the main field, 
the motor running as a series machine with an ex¬ 
cessive backward lead, thus causing destructive 
speed. In practice, fuses would protect a motor 
from injury, because the fuses, if of proper size, 
would blow from excessive current before the motor 
attained a destructive speed. A commutating-pole 
shunt motor subject to sudden heavy loads is more 
liable to hunt than is a similar motor subject to a 
steady load. A sudden load increases the com¬ 
mutating field suddenly, while the main field is un¬ 
changed, thereby causing an unbalancing of the 
fields. A motor once properly adjusted will run 
indefinitely with good results. 

Commutators should always be turned and put 
in good condition when introducing the commu¬ 
tating-pole construction. The brushes of a com¬ 
mutating-pole motor should never be moved after 
once being set. In some special cases where a 


158 


Brushes 


motor runs only in one direction, a slight forward 
lead makes a motor more stable. The brushes of 
a commutating-pole generator may be moved 
slightly backward or forward as far as is consistent 
with good commutation in order to give the neces¬ 
sary voltage change. 

The addition of commutating poles opens the 
way to run motors on higher speeds or generators 
at reduced voltage and full-load current by weaken¬ 
ing the main field while retaining good commutation. 
The field of commutating-pole motors should never 
be weakened until two very important factors are 
accounted for. The armature and the commu¬ 
tator should be able to withstand the added centrif¬ 
ugal strains, and the motor should be carefully 
tested for hunting at the extreme weak field con¬ 
dition and run at not less than fifty per cent, over¬ 
load. Armatures and commutators as usually 
constructed should not be run at a peripheral speed 
of over 5000 feet per minute. 

When the foregoing suggestions are carefully 
followed, machines that have previously given 
trouble from poor commutation or excessive arma¬ 
ture heating will run like new, and repay many times 
over the cost of adding the .commutating poles by 
a lower energy consumption and a lessened main¬ 
tenance charge. 


159 


CHAPTER XX 


CURVES 

How Curves are Plotted and Their Usefulness — 
Saturation Curves — Characteristic Curves of 
Shunt, Series, and Compound Wound Generators 
and Applications for Which Each is Particularly 
Adapted . 

A glance at the outline of a man’s face drawn 
on paper, tells to a large degree many things about 
him. So it is with an indicator card taken to test a 
steam engine; its curving outlines reveal to the 
experienced observer the inner workings of the 
engine showing whether or not it is running correctly 
and economically. Also in electrical machines 
curve sheets can be prepared w T hich show a complete 
catalogue of the attainments of any piece of ap¬ 
paratus. In a motor, for example, a curve can be 
made to show the speed at any given load or the 
current for any speed, etc., and in a generator the 
volts at any current output or field strength. To 
understand the reading and drawing of these curves 
let us take a simple case showing the relation be¬ 
tween kilowatts and horsepower. In this instance 
the curve will be a straight line. Curves drawn to 
show a fixed relation between two things such as 
kilowatts and horsepower are always straight lines. 

Fig. 168 is drawn as follows: Take a sheet 
ruled into regular squares. Paper so prepared is 
called “cross section paper” and can be readily 
purchased; T V' or ^V' squares are best, although 
paper is sometimes used with millimeter squares. 
Distances measured along the base of sheet are 


160 


Curves 


called “abscissas” and distances measured in a 
vertical direction are called “ ordinates.” In Fig. 
168 we call base line horsepower and vertical line 
kilowatts. Starting at A, horsepower is zero, there¬ 
fore kilowatts are zero. A is then a point on curve 
as it shows a relation between horsepower and kilo¬ 
watts. Now select any value for horsepower as 20, 
then 20 X .746 = 14.9 kilowatt. On the 20 H.P. line 
lay off 14.9 KW. and a new point B is determined. 
Again select 50 H.P. and lay off on this vertical line 



Fig. 168. — Curve Showing Relation Between Horse¬ 
power and Kilowatts. 


50 X .746 = 37.3 KW. and point C is located. Again 
start with 30 KW. and lay off on horizontal 30 KW. 
line, 40.3 H.P. (30-f->746 = 40.3) and still another 
point D is found. Joining A, B,C, and D, a curve is 
found which shows relation between kilowatts and 
horsepower, or vice versa. From this, values of 
kilowatts or horsepower can at once be inter¬ 
changed. For example, to find how many kilowatts 
in 30 horsepower, look up 30 horsepower line until 
it cuts curve and note what KW. line intersects with 
curve at same point. In this case it is 22.4 kilo¬ 
watts. This curve could just as well have been 


161 






















Electricity and Electrical Apparatus 

drawn using kilowatts as abscissas and horsepower 
as ordinates. 

Another simple case is Fig. 169 showing rela¬ 
tion between horsepower and speed of a ship in 
miles per hour. It is well known that after a ship 
gets up to normal speed, any increase in speed re¬ 
quires an abnormal horsepower and consequent use 
of coal. In Fig. 169, the abscissas are horsepower 



Fig. 169.—Curve Showing Relation Between the Speed 
of a Ship and the Power of its Engines. 


and the ordinates miles per hour. Tests of a ship 
show the following results: 


10 miles per hour 



25 “ “ “ 


1000 Horsepower 
1800 
3000 
5500 
8000 


In Fig. 169 draw the different miles per hour 
lines, also draw the different horsepower lines. The 
intersections of corresponding values will be points 
on curve, which shows the relation between any 


162 




























Curves 


speed and horsepower for this particular ship. For 
example, 4000 H.P. would drive this ship 22 miles 
per hour. 

The results of the following experiment may be 
plotted. Arrange apparatus as shown in Fig. 170 
and allow different noted currents to pass through 
coil, meanwhile measuring the pounds pull necessary 
to separate bars. Reading as follows: 

Current in Amperes Pull in pounds 


1 

2 

3 

4 

5 

6 
7 


8.5 

13.5 

16.5 

18.5 

19.6 

20.5 

20.6 


The iron becomes saturated at about 5 amperes 
in coil, that is, any further increase in current to 
gain added magnetic effect is wasted. Laying out 
the results of this test is called “plotting the points.” 
The curve, Fig. 171, shows pull for any given current, 
or vice versa. If cast iron bars were substituted in 
place of the mild steel the magnetic pull would be 
much less and would be shown by the lower curve 
in Fig. 171. This method of determining satura¬ 
tion is very crude, and more elaborate methods are 
resorted to in laboratory work. The reluctance or 
magnetic resistance of iron and steel varies with the 
flux density. In sheet iron or steel when density 
gets up to 95 or 100,000 lines per square inch and in 
cast iron when density reaches from 35 to 40,000 the 
reluctance increases very rapidly. For example, 
it requires twice the magnetizing energy, which 
means double the number of ampere turns, to force 
flux through a given cast steel bar at 100,000 density 
than for a density of 90,000. Magnetic materials 
are said to be saturated when the reluctance be¬ 
comes excessive. 


163 


Electricity and Electrical Apparatus 


Fig. 172 shows saturation curves of a standard 
grade of cast steel, sheet steel and cast iron. These 
curves are plotted from laboratory data giving am- 



Am meter. 




Fig. 170. — Electro-Magnet with Core and Armature of 
Steel or Cast Iron. 

pere turns per inch length of magnetic circuit for 
abscissas, and number of magnetic lines of force per 
square inch for ordinates. In a magnetic circuit it 
requires a certain number of ampere turns to force 
a certain number of lines of force per square inch 
through one-inch length. For example, a cast-steel 
bar one inch square, four inches long, requires 26.0 
ampere turns per inch to drive 80,000 lines of mag¬ 
netic force through it, or a total of 26 X 4 = 104 
ampere turns. 


1G4 




















Curves 


To drive lines of force through air requires a 
fixed number of ampere turns per inch for a given 
density. Fig. 173 shows this relation. With curves 
shown in Figs. 172 and 173 the ampere turns neces- 



Fig. 171. — Curves Showing Relation Between Current 
and Pull Necessary to Detach the Armature, 
for'Steel and Cast Iron. 


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Fig. 172. — Magnetization Curves of Wrought Iron, 
Cast Steel, Cast Iron, and Sheet Iron such as 
Used for Armature Punchings. 


165 







































































Electricity and Electrical Apparatus 

sary to drive a given flux through any magnetic 
circuit can be calculated. 

Generators are designed to operate at approxi¬ 
mately constant speeds, and characteristic curves 
of generators are generally limited to showing re¬ 
lation between voltage at terminals and current 
output. 

Series generators have widely varying charac¬ 
teristics. Fig. 174 shows relation between am¬ 
peres and volts on a series generator. Curve OA 



Fig. 173. — Magnetization Curve of Air. 


shows voltage generated by armature, while OB is 
volts drop in armature, field and brushes, due to 
resistance. Subtracting OB from OA gives OC, 
the available terminal voltage. Note that the vol¬ 
tage of a series generator changes with load. Ma¬ 
chines of this type when operated with external 
regulators can be made to give an approximately 
constant current for series arc lighting. This 
method of lighting is not so widely used as formerly. 
The commercial adoption of series generators is 
restricted, the only other use of note being their 


1GG 
























Curves 


employment as boosters, principally in street rail¬ 
way plants. 

In Fig. 175, the booster shown is driven by a 
motor and in this way adds 50 volts to the genera¬ 
tor terminal pressure. 

Fig. 176 illustrates another method of con¬ 
necting a booster, so that it carries only part of 
generator current, and provides 550 volts at the 



Fig. 174. — Characteristic Curve, Series Generator. 


end of line, making the pressure more uniform 
throughout the system. 

Boosters should preferably be direct connected 
and always provided with a centrifugal shaft switch 
so arranged as to disconnect booster from circuit 
if it exceeds a certain speed. Any loss of driving 
power will allow the current to stop the booster, 
start, and drive it in the reverse direction as a motor. 
Without a centrifugal switch it will run away and 


167 



































Electricity and Electrical Apparatus 

be thrown to pieces, being practically a 50-volt 
series motor without load on a 550-volt circuit. 

Fig. 177 shows curves for a separately excited 
shunt generator. Curve OA represents voltage 



Fig. 175. — Series Generator Used as a “Booster.” 

generated if no RI drop. Curve OB is RI drop in 
armature and brushes. OC, the difference between 
OA and OB, is voltage at terminals. Separately 
excited shunt generators are seldom used except 


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Fig. 176. — Series Booster Connected to Long-Distance 
Feeders Only. 

for low voltage plating work. A curve for a self- 
excited shunt generator is shown in Fig. 178. Note 
that at point D the field is so weakened by reduction 
of voltage that it ceases to supply sufficient flux to 
keep up voltage and generator “dies down” or 


168 




























Curves 


ceases to generate. In a commercial machine this 
falling off point is at such a heavy armature current 
that machine will not show this action except on 
dangerous overloads. 

Shunt generators are used in small sizes 
from 3 to 5 KW. where variation of voltage due to 
change of load is not a detriment or where voltag'e 
can be regulated by hand. Shunt generators of 



Fig. 177. — Characteristic Curves, Separately 
Excited Generator. 


larger capacity can be used on constant loads, such 
as hotels or large stores, where change of load is 
gradual, and can be met by attendant at switch¬ 
board field rheostat. Compound wound generators 
require no attendant at rheostat, due to their auto¬ 
matic regulation of voltage, except when connecting 
one generator in parallel with another. 

In a compound generator we have a series field 
supplying the ampere turns lost by main shunt 


169 























Electricity and Electrical Apparatus 

field when a reduction of voltage occurs, due to RI 
drop in lines or the increasing armature reaction 
as load comes on. That is, the series winding helps 
out the shunt field. 

In Fig. 178 OA is voltage generated by shunt 
field flux, assuming no RI drops, OE voltage gene¬ 
rated by both shunt and series field flux, assuming 
no RI drops. If now the RI drops shown in OC be 
subtracted from the curve OE, a voltage OB will 
result. By changing the strength of series field this 
line OB can be made to rise or fall, that is, give a 



Fig. 178. — Characteristic Curves, Shunt and 
Compound Generators. 


greater or less voltage as load comes on, making 
a “rising” or “ lowering” characteristic. Com¬ 
pound generators are always used for supplying 
varying loads, as street railway service, electric 
elevators, etc. They are now almost universally 
used for all sources of direct current except for 
special generators, such as low voltage platers or 
high voltage series machines. 


170 

























CHAPTER XXI 


ALTERNATING CURRENTS 

Explanations of Alternating Currents and Some of 
the Differences from Direct Current — Curves 
of Alternating Current—Single Phase — Poly¬ 
phase — Two and Three-Phase Current — Delta 
and Y-Connection — Alternators. 

An Alternating Current, as indicated by its 
name, is one that periodically alternates in its di¬ 
rection of flow. The nature of the alternating cur¬ 




rent, especially in comparison with direct current, 
may well be illustrated by reference to Figs. 179 
and 180. 

Referring to Fig. 179, let us suppose the cylinder 
and coil of pipe to be filled with water, and the piston 


171 






































Electricity and Electrical Apparatus 

P, actuated by the rod R, to be moved toward the 
left. This will cause the water to flow from the left 
of the cylinder into the right through the coil of pipe 
C in the direction indicated by the arrow. 

Now let us suppose that the piston reaches the 
end of its travel and starts on the return stroke. 
Immediately the water has to flow from the right 
end of the cylinder to the left through the coil C in 
the direction opposite to that indicated by the arrow. 
In a similar manner, alternating current flows first 
in one direction and then in the other. 

Now let us consider Fig. 180, which shows a 
similar analogy of the direct current. If the piston 
is moved toward the left, the valves A, B, C , and D 
are so arranged that B and C open and A and D 
close and permit the water to flow through the coil 
in the direction of the arrow, as in Fig. 179. When 
the piston starts on the return stroke, while B and 
C close, A and D open, permitting the water to 
continue to flow in the same direction through the 
coil C. As the action of Fig. 179 is simpler than 
that of Fig. 180, even so is the action of an alterna¬ 
ting current generator simpler than that of a direct 
current generator; but as the characteristics of the 
water flowing through the coil in Fig. 179 are more 
complicated than those of the water flowing through 
coil in Fig. 180, even so the characteristics of alter¬ 
nating currents are more complex than those of 
direct current. 

The valves A, B, C and D serve the same pur¬ 
pose in Fig. 180 as the commutator on a direct current 
generator. Again referring to Fig. 179, let us sup¬ 
pose the piston P, being at the end of its travel to¬ 
ward the right, to move to the end of its travel to 
the left, then back again to its original position at 
the right. The water in the coil C will have started 
to flow in the direction indicated by the arrow, 
stopped, and returned in the opposite direction 
until it has the position it occupied before the piston 


Alternating Currents 

was moved. The water may be said to have gone 
through a complete cycle of operations. In a 
similar manner when an alternating current has 
started at zero, reached a maximum value, decreased 
to zero, increased to a maximum in the opposite 
direction, and again returned to zero, it is said to 
have gone through one complete cycle. 

The number of complete cycles through which 
an alternating current passes in one second is called 
its frequency. 

The frequencies most commonly found in light¬ 
ing and power plants in the United States are 25 
and 60 cycles, although there are a few of the older 
stations still operated at 125 and 133 cycles, and 
some of the recent installations in connection with 
street railway work employ 15 cycles. Very much 
higher frequencies than these are used in wireless 
telegraph work. 

Any conductor having a current of electricity 
flowing through it in a certain direction will have 
a magnetic field set up around it in a corresponding 
direction. If this current is reversed, the magnetic 
field will also be reversed. 

Consider the piston P in Figure 179 to be mov¬ 
ing towards the left and the water flowing through 
the coil C in the direction indicated by the arrow. 
Suppose a force be exerted on the piston rod R to 
force the piston P toward the right. Before P can 
move toward the right the force must stop the water 
flowing in the coil C and start it flowing in the 
opposite direction. The inertia of the water not 
only tends to prevent this but tends to keep the 
piston moving towards the left. 

In a similar manner, the magnetic field around 
a conductor carrying current not only tends to pre¬ 
vent the reversal of that current, but to continue 
its flow in the same direction. Let us suppose that 
the piston P and the rod R are without weight, that 
the external force applied to R is the voltage or 


173 


Electricity and Electrical Apparatus 

pressure of the circuit, the piston P to be the current 
flowing and the water in C to be the magnetic field 
set up around the conductor due to the current 
flowing. Then even as the external force has to be 
applied to R an appreciable length of time before the 
direction of flow of the water in C is reversed, so the 
voltage or pressure of the circuit has to be applied 
an appreciable length of time before the current is 
reversed. In other words, the current lags behind 
the voltage. This tendency to keep the current 
flowing in the same direction is the effect of in¬ 
ductance of the circuit and has to be taken into con- 



Fig. 181. 


sideration in calculating line losses and the amount 
of voltage necessary to force a given amount of 
alternating current through any circuit. 

As applied to direct current, Ohm’s Law is 
E 

> where / is the current, E the voltage, and 

R the resistance of the circuit. The same law ap¬ 
plies to the alternating current, only for R , the 
resistance of the circuit, we must substitute the 
impedance of the alternating current circuit, which 
is the combined resistance and reactance, and is 
represented by VR 2 + X 2 , where R is the ohms 
resistance and X is the reactance. The reactance 


174 














Alternating Currents 


is the inductance multiplied by a constant depend¬ 
ing on the frequency. 

E 

Thus the law becomes I = — — • 

VR 2 + X 2 


Where condensers are used, or the circuit is such as 
to give a condenser action, there is another factor 
affecting the flow of an alternating current. This 
factor is called the condenser or capacity effect of 
the circuit, and when it is taken into consideration 


the law becomes 



where C is 


the capacity effect. 

A comparison of this formula with that for 
direct current will give a good idea of how much 
more complicated the action of an alternating cur¬ 
rent appears when compared to direct. 

In Fig. 181 is shown a single loop of wire ar¬ 
ranged to conveniently revolve between the poles 
of a permanent magnet. This may be considered 
the simplest form of an alternating current gene¬ 
rator or alternator. As the loop rotates in the 
direction indicated by the arrow, it will cut the 
lines of magnetic force between the two poles of 
the magnet. The induced electromotive force will 
tend to send a current through the loop in the di¬ 
rection indicated by the arrow heads on the loop. 

Let us suppose that the ends of the loop are 
connected to collector rings; and brushes, con¬ 
nected externally by the wires and lamp, bear 
upon the collector rings. Then a complete electrical 
circuit will be formed through the revolving loop, 
the collector rings, the brushes and the external 
circuit. Consequently, as the loop is caused to 
revolve, a current will be set up in the direction 
indicated, which will be a maximum when the loop 






Electricity and Electrical Apparatus 

is in the position shown, and will gradually decrease 
until the loop has passed through 90°, when the 
current will be zero. This may be readily under¬ 
stood when it is seen that at this position the sides 
of the loop are moving parallel to the lines of mag¬ 
netic force and consequently do not cut them. 

Now suppose that the loop continues to revolve 
in the same direction. The two sides of the loop 
will be cutting lines of magnetic force in the opposite 
direction to what they were before, consequently 
a current will flow through the loop in a direction 
opposite to that indicated by the arrow heads. This 



Fig. 182. — Wave of E.M.F., Produced by a Single Coil. 


current will gradually increase until the loop has 
completed a half turn from the position indicated 
in Fig. 181, when it will be a maximum. As the 
rotation is continued the current will decrease and 
again pass through zero as the loop revolves through 
a point 270° from the starting place in Fig. 181. 
On the completion of one revolution and the return 
of the loop to its original position the current will 
have again reached the maximum value of flow in 
the direction indicated in figure. The alternating 
current has thus passed through one complete cycle. 


176 

























Alternating Currents 


Considering only the top side of the loop, the 
whole cycle may be graphically illustrated by Fig. 
182, the accompanying curve showing correspond¬ 
ing relative values of the electromotive force in¬ 
duced or the current flowing. The smali circles 
indicate the various positions of the conductor with 
respect to the magnetic poles. 

If instead of having one turn the loop should 
have several turns distributed over a considerable 
portion of the circumference of the circle in which 
it revolves, we would have conditions which would 
give a more gradual change to the direction of flow 
of the current. This is readily understood when it 



Fig. 183. — Sine Wave. 


is considered that all of the conductors would not 
come under the influence of a pole at once, but one 
at a time. Under these conditions the values of 
the E. M.F when plotted would nearly approximate 
the sine wave. A true sine wave is shown in Fig. 
183. It is convenient to assume in calculation that 
the wave is of this form. 

Thus far we have been considering the simplest 
form of alternating current, known as Single Phase. 
This form is used principally for lighting and small 
motors, while two and three phase is employed for 
large motors and heavy power, and for long distance 
transmission. 


177 * 



Electricity and Electrical Apparatus 


The terms, Single, Two or Three Phase, mean 
that the machine in question (motor, generator, or 
transformer, as the case may be) has one, two, or 
three separate and distinct windings. 

Several questions naturally arise in your mind: 



Fig. 184. — Three-Cylinder Pump. Analogy of a 
Three-Phase Generator. 

Why are two or more windings better than one, if 
the same amount of wire is used? How are the 
phases connected, and what relation do they have 
to each other? When is single phase used in pref¬ 
erence to three phase, or three phase instead of 
single phase, and why? All of these questions will 


' 178 






















Alternating Currents 

be discussed later, but at present we must learn 
the nature of polyphase currents; that is, currents 
of more than one phase. In doing so we shall find 
at least a partial answer to these questions. 

We will first draw an analogy between three- 
phase alternating current and a pump with three 
cylinders. You are familiar with the gushy, pulsa¬ 
ting operation of a single cylinder pump. If three 
cylinders were so arranged that the pistons were 
driven by cranks on the same shaft, spaced 120° 
apart or each one-third of a revolution from the 
other, the three strokes would come one after the 
other in regular succession, and result in a more 
even and much steadier flow from the outlet. 

It looks reasonable, and as a matter of fact it 
is true, that the power obtained from a single phase 
winding is not steady, but pulsates, becoming zero 
every time the current or voltage wave passes through 
zero. To be sure, the current alternates so rapidly 
that any motor driven by it would not stop during 
the interval of no power, and the higher the fre¬ 
quency the less noticeable would be the effect of 
this short interval. But for large, expensive ma¬ 
chines it is better to avoid any such jerky, vibratory 
action, if possible. 

As in the case of the three-cylinder pump, if 
three windings are so arranged in the armature of 
an alternator that the currents generated by them 
come to their maximum value 120° or one-third of 
a cycle apart, one after the other in regular suc¬ 
cession, the source of power is more steady and con¬ 
stant. 

In Fig. 185 are shown the initial position of the 
armature coil of phase A, and the curve of E.M.F. 
produced by the coil as it rotates. It will be noted 
that the winding is shown directly under the center 
of the pole, where it will cut the greatest number of 
lines of force. Therefore, the current generated 
will be a maximum at this point in the revolution. 


179 


Electricity and Electrical Apparatus 

In the diagrams, OA and OB represent positive 
and negative values of the current, while points 
along the line OX indicate the position of the arma¬ 
ture coil, or rather the amount it has been rotated 
from the initial position shown. 

Figs. 186 and 187 show the initial positions 
and the E.M.F. curves produced by the coils of 
phases B and C respectively. It will be noticed 
that the coils are shown in positions 120° from each 
other, and that phase B will reach its maximum 120° 
or one-third of a revolution later than A, and C 




Fig. 187. — Phase C Fig. 188.—E.M.F. Curves of 
a Three-Phase Generator. 


120° later than B, or one-third revolution before A 
reaches its second maximuni. 

Fig. 188 represents the three coils on the 
same armature, and their curves all drawn on the 
same diagram. This diagram shows that some 
phase of the machine is generating an E.M.F. in 
the positive direction all the time. It can be proved 
that the total power, which is the sum of the power 
outputs of all three phases, does not pulsate, but is 
constant at all parts of the cycle. 

The currents from each phase of a three-phase 
generator might be conducted away by a separate 
pair of wires, making six wires in all; but in practice 


180 
































Alternating Currents 


it is unnecessary to use more than three wires. The 
reason for this may be understood by referring again 
to Fig. 188. The curves may be considered to rep¬ 
resent current instead of E.M.F., as usually the 
current wave has nearly the same form as that of 
E.M.F. If a vertical line be drawn through the 
curves at any point, it will be seen that the sum of 
the distances to the curves above, along this line, 
from the zero line (OX), is always equal to the sum 
of the distances to the curves below the line OX. 
That is, the current from one phase flowing out along 



Fig. 189a. Fig. 189b. 

Delta (A) and Star or Y-Connections for 
Three-Phase Circuits. 


one wire returns in the form of the currents from 
the other two phases, which are flowing in toward 
the generator along the other two wires. 

That it may do this, evidently the three wind¬ 
ings must be connected together in some way, both 
at the generator and at the motors or other ap¬ 
paratus where the electric energy is to be used. There 
are two methods of making these connections, as 
illustrated in Figs. 189a and 189b. In Fig. 189a, the 
phases are said to be Delta or A-connected, from the 
resemblance of the diagram to the Greek letter A, 
and in Fig. 189b they are Star or Y-connected. 

In the A-connected form, the voltage between 
the line wires is the same as that generated in each 


181 






Electricity and Electrical Apparatus 

phase winding, but the current in each wire is that 
which flows through the two phase windings that 
are connected to that wire. This does not mean 
that the amount of current in the wire A, for ex¬ 
ample, is twice as much as that in the winding b 
or c. To see why this is not the case, let us refer 
to Fig. 190. Curve b is the current in the winding 
b of Fig. 189a, and curve c is that in the c winding, 
which comes to a maximum 120° later. As the 
currents from both b and c flow in the wire A, the 
curve A, which represents the sum of the currents 



Fig. 190. 


b and c at each instant, will represent the current 
in wire A. You will note that while this current is 
greater than either b or c, it is not twice as great. 
As a matter of calculation, it is equal to the current 
in b times the square root of three (V3 ) or 1.732. 

In the F-connected circuit, on the other hand, 
evidently the current in wire A is the same that 
flows in phase a. But the voltage between the 
wires A and C at each instant is the sum of those 
generated in phases a and c. Fig. 190 may be con¬ 
sidered to represent these voltages in this case just 
as they represent currents in the case of A con¬ 
nections. Therefore, the voltage between wires is 
that of each phase multiplied by Vs. 


182 





Alternating Currents 


Two-phase systems are in some respects simpler 
and easier to understand than three-phase, but there 
are many disadvantages compared with three-phase. 
One is that four wires must be used instead of three, 
except at a great disadvantage in the voltage reg¬ 
ulation. While there are many two-phase systems 
in existence, the tendency is toward three-phase 
systems for power, and for long distance trans¬ 
mission of current for lighting purposes. In some 
systems using large synchronous motors or rotary 
converters, 6 and even 12-phase is used; but of late 
years the best practice has eliminated these com¬ 
plicated windings, and now the tendency is toward 
an almost universal use of three-phase for large 
power. 

Examine an ordinary incandescent lamp socket 
and you will note that there are but two connections. 
Evidently only one phase can be used for lighting. 
For this reason single-phase current is necessary 
for supplying lights, but they are often connected 
to a three-phase system in such a manner that one- 
third of a given group of lights is supplied from each 
of the three phases. To accomplish this, one-third 
is connected between wires A and B, one-third be¬ 
tween B and C, and one-third between A and C. It 
is well to have the load balanced as nearly as possible 
between the phases, as the regulation of voltage is 
poor if one phase has considerably more load than 
another. 

In recent years alternating current has been 
tried quite extensively to supply power for electric 
railways. On account of the difficulty of using 
more than one trolley wire or extra rail, it is neces¬ 
sary to use single-phase current for such purposes. 

Alternating current generators are usually 
termed Alternators, to distinguish them from direct 
current generators. As an A.C. machine does not 
need a revolving commutator, alternators are 
usually built with the armature in the frame of the 


r 


Electricity and Electrical Apparatus 

machine, the field magnets forming the rotating 
element. This makes it necessary to pass only a 
comparatively small current, to excite the field, 
through the brushes and sliding contacts, which 
are always a source of more or less trouble when 
carrying large currents. A revolving field alterna¬ 
tor is illustrated in Figs. 116, 117 and 118. 

Looking back over this chapter, we see that: 

1. Alternating-current generators and motors 
are mechanically much simpler than those for direct 
current. 

2. The characteristics and behavior of alterna¬ 
ting current is much more complex to calculate. 

3. The current alternates in the form of waves, 
very nearly like the so-called sine wave. 

4. Alternating-current systems are Single- 
Phase, or Polyphase. Polyphase systems generally 
used are Three-Phase, or Two-Phase. Six. or even 
Twelve-Phase, is sometimes used. 

5. The power delivered by a polyphase gene¬ 
rator does not pulsate. 

6. Three wires are used instead of six to carry 
three-phase current. The three phases may be 
either A or Y-connected. 

7. Single-Phase is used for lights and small 
motors, and for alternating current railway systems. 
Polyphase, and usually, in recent years, Three-Phase, 
is used for large power and long distance transmission. 


184 


CHAPTER XXII 


TRANSFORMERS 

Development — Constant Potential Type — Simple 
Alternating Current Transmission Line and 
Explanation Why and How Alternating Current 
can he Transformed — Reasons for Use of 
Alternating Current for Long Distance Trans¬ 
mission Work — Constant Current Transformers. 

Electrically speaking a transformer is a piece of 
apparatus for changing the voltage of an alternating 
current circuit. Essentially a transformer consists 
of two coils of wire or other electric conductor sur¬ 
rounding a core of iron or other magnetic material. 
Through one of these coils an alternating current is 
caused to flow; this is called the primary coil or 
simply the primary. From the other coil, current 
may be supplied for lighting or other purposes; this 
is called the secondary coil or simply the secondary. 

A simple transformer is illustrated in Fig. 191. 
If a current is caused to flow in the primary, mag¬ 
netic flux will be set up in the iron core in a certain 
direction. Now we cannot conceive of these mag¬ 
netic lines circulating around the iron without be¬ 
ing cut or interlinking with the turns of the second¬ 
ary coil. This induces an E.M.F. in the secondary 
and if the ends of the coil are electrically connected 
a current will flow. If the primary circuit is opened 
and current ceases to flow the magnetomotive force 
will be withdrawn and the magnetic flux will cease 
to exist in the core. Again we cannot conceive of 
this magnetic flux being withdrawn without causing 
a change in the number of interlinkages with the 


185 


Electricity and Electrical Apparatus 

secondary turns. This will again set up an E.M.F. 
in the secondary but in the opposite direction from 
the first case. Now let us suppose a current is 
caused to flow in the primary in the opposite 
direction to the original, a flux will be set up in the 
core in the opposite direction, inducing an E.M.F. 
in the secondary in the same direction as when the 
first magnetic flux was removed. As the current in 
the primary again dies down to zero, this second 
magnetic flux ceases to exist, and an E.M.F. will be 
induced in the secondary in the same direction as 
when the first magnetic flux was set up in the core. 
We may now consider the transformer to have passed 



Fig. 191. — A Simple Transformer. 


through one complete cycle. If the primary is 
connected to an alternating current source of supply, 
the reversals of the alternating-current will cause 
similar reversals in the magnetic flux, inducing an 
alternating E.M.F. in the secondary. With the 
continuous changing in value of the alternating- 
current, there is a corresponding variation in the 
amount of flux set up by the primary and inter¬ 
linking with the secondary. And it is upon this 
never-ceasing variation in the number of inter¬ 
linkages that the action of a transformer depends. 
If connected with a direct-current circuit, there 
would be a voltage induced momentarily in the 
secondary as the core flux is rising to its ultimate 


186 










Transformers 


value. As soon as this ultimate constant value is 
reached the number of secondary interlinkages be¬ 
comes constant and the induced voltage becomes 
zero. Thus we see a transformer of this kind is not 
applicable to direct-current. Rotary converters 
or motor generator sets (sometimes called rotary 
transformers) must be used for changing the pres¬ 
sures of direct-current circuits and even with these 
more or less complicated devices the range of avail¬ 
able pressures is limited. 

To prevent eddy currents, large core losses and 
excessive heating, transformer cores are built of 


r - 

1 

— 

1 

A 


L 

— 



She// ~Type 
Fig. 192. 


r~ 

— 

1 

r - 


B 


L_ 


J 

L. 


C o A-e /jy pG 

Fig. 193. 


laminations. There are two general forms of trans¬ 
former cores, known as the core type and shell type 
(See illustrations). 

Fig. 194 shows transformer made by Westing- 
house Electric ajid Manufacturing Company in 
smaller sizes and suitable for pole suspension. The 
corrugated form of case or box used increases the 
radiating surface and allows the transformer to 
operate at a lower temperature. Also by this con¬ 
struction a stronger case can be made with a given 
amount of iron. The interior view of a Moloney 
core type transformer is shown in Fig. 195, while the 
interior of the large shell type transformer made by 


187 



















Electricity and Electrical Apparatus 


the Allis-Chaimers Company is shown in Figs. 196 
and 197. 



Fig. 194. —Westinghouse Trans- Fig. 195. — A Moloney 
former in Corrugated Case. Core Type Transformer. 
































Transformers 


Fig. 198 illustrates the core and coils of a trans¬ 
former put on the market by the Western Electric 
Company. In place of core type they have adopted 
as their standard of production the cruciform core, 
having four magnetic circuits of equal reluctance. 
The central leg of the core is covered with fibrous 
insulation and tape to protect the windings from 



Fig. 198. — End View of Core and Coils, “ Hawthorne ” 
Type T Transformer. Western Electric Co. 

chafing on the rough edges of the laminations, and 
to prevent grounds. 

The winding machines are similar to lathes. 
In the smaller sizes the central leg of core is itself 
placed in the winding machine and the secondary 
and primary coils wound directly on top of the fibre 
and tape insulation, mica shields being placed so as 
to prevent any electrical connection or short circuit 


189 











































Electricity and Electrical Apparatus 

between the primary and secondary windings. Fig. 
198 shows a larger size, in which the coils are wound 
on forms and placed on the core, the secondary be¬ 
ing divided into two coils, one placed each side of 
the primary. 

After the winding is completed the outside 
portions of the four magnetic circuits are put into 



Fig. 199. — “ Hawthorne ” Type T Transformer Coils 
and Cores, Entering the Insulating Compound 
Impregnating Tank. Western Electric Co. 

place and the complete core, coils and connection 
board are put through a vacuum drying and com¬ 
pound filling treatment which is worthy of mention. 
The fibrous insulation used in the Western Electric 
Company transformer is sufficient to stand high 
potential tests for all ordinary conditions of safety. 
When the core, coils and terminal-boards com¬ 
pletely assembled are placed in this compound tank, 


190 






Transformers 

shown in the illustration, they are first subjected to 
a very thorough baking, until all moisture is ex¬ 
pelled from even the innermost parts of the wind¬ 
ings. The coils are then sealed up by the compound, 
which is forced in under pressure while in a molten 
condition. Upon solidification of this compound 
the coils become moisture proof and are also given 



Fig. 200.—Assembled View of Core, Coils, Leads and 
Terminal Board of “ Hawthorne ” Type T 
Transformer. 

additional mechanical stability and protection from 
chafing. This tendency to chafe is due to vibra¬ 
tion from the alternating current and to the alter¬ 
nate expansion and contraction taking place as the 
transformer heats and cools. Also it is claimed 
that this compound, being a good conductor of heat, 
insures lower operating temperature than can be 
obtained without its use. 

191 






Electricity and Electrical Apparatus 

Figs. 200 and 201 show views of Western Electric 
transformer before and after coils have been placed 
in case. 

The Westinghouse Electric & Manufacturing 
Company have a modified shell type transformer 
with a core shaped very similar to the cruciform in 
the Western Electric Company type. 

Fig. 205 shows the assembled coils for two 
different Westinghouse transformers, bringing out 



Fig. 201. — External View, “Hawthorne” 

Type T Transformer. 

very clearly the method used to separate the coils 
and to provide ventilating ducts through which the 
oil circulates. This is a very important feature as 
it offers a means for cooling the inside of the coils 
and prevents excessive temperature near the core 
while the outside might apparently be cool. 

As oil is a much better conductor of heat than 
air, the operating temperature of the transformer 
will be reduced if the coils are immersed in oil. An 
oil commonly used for this purpose is known as 


192 








Transformers 



Fig. 202. — Westinghouse Type S Transformer. 



Fig. 203. — Leaf Connector Used on Large 
Westinghouse Transformers. 



Fig. 204. — Magnetic Circuit of Westinghouse 
Type S Transformer. 

193 





Electricity and Electrical Apparatus 

“Transil Oil,” No. 6 and No. 8 being two grades in 
commercial use. It is obtained by fractional dis¬ 
tillation of petroleum, unmixed with any other sub¬ 
stance and without subsequent chemical treatment, 
and rather than having any injurious effect upon 
the insulation, its influence should be preservative, 
making the insulation soft and pliable, increasing 
the insulating value of the transformer and pre- 



Fig. 205. — Assembled Coils of Westinghouse Type S Transformers, 
Showing Ventilating Ducts. 


venting oxidization by the air. It should have a 
high flash point, or in other words, the temperature 
at which it will ignite should be high, and also its 
freezing point should be reasonably low, so as not 
to freeze in winter. 

Although counteracted to a certain extent by 
the heat from the windings, transformers are fre¬ 
quently located out of doors and subjected to severe 
cold. The freezing point of No. 6 transil oil is 12 


194 






Transformers 


degrees below zero (Centigrade) and for No. 8 it is 
18 degrees below zero (Centigrade). 

The ratio of the primary voltage to the second¬ 
ary voltage is called the ratio of transformation, or 
simply its ratio. Instead of quoting the actual 
voltages of transformers it is customary to speak of 
the ratios as so many to 1. That is a transformer 





Fig. 206; — Allis-Chalmers Oil-Filled, 
Water-Cooled Transformer. 

A coiled pipe is placed in the oil. Cold water flows through the 
pipe and keeps the operating temperature at a low point. 

ratio 2,000 volts to 100 volts would be called ratio 
20 to 1. Now let us suppose we have a transformer 
with a ratio 1 to 1. There the primary and second¬ 
ary voltages would be the same and the primary 
current would equal the secondary current plus a 
small amount of extra current, which is commonly 
known as the exciting current, and still another small 
amount of current that supplies the energy lost in 


195 





Electricity and Electrical Apparatus 

the apparatus. For the present we will disregard 
the losses of the transformer. 

The alternating flux in the core not only induces 
an alternating E.M.F. in the secondary but also in¬ 
duces an alternating E.M.F. in the primary, which 
is always in opposition to the impressed E.M.F. 
The magnetizing current producing the magnetic 
flux increases until this induced or back E.M.F. is 
nearly equal to the impressed E.M.F. This back 
E.M.F. prevents an excessive current from flowing 
in the primary, the resistance of which is very low. 
Now if the secondary supplies current to a load, say 
of lamps, this secondary current will tend to set up 
a magnetic flux in the core in the opposite direction 
to that set up by the primary current. This at once 
causes the flux set up by the primary to decrease, 
causing the induced or back voltage in the primary 
winding to decrease, which will allow a greater 
primary current to flow and consequently tend to 
bring the magnetic flux up to its first value. Thus 
as the load or amount of current taken from the 
secondary is increased, the primary automatically 
takes an increased amount of current from the source 
of supply. 

The advantages of the transformer are synony¬ 
mous with the. advantages of the alternating cur¬ 
rent systems; that is, the economy in distribution 
and long-distance transmission of power. 

With the exception of the special cases with 
ring-armature construction, it is not practicable to 
build a direct-current machine for greater than about 
six or seven hundred volts. At this latter voltage, 
the expense of transmitting power any great dis¬ 
tance would be prohibitive. 

Let us suppose we had to deliver a certain 
amount of water per hour over a distance of one 
mile. We could deliver this water through a large 
pipe at low pressure or through a small pipe at high 
pressure. In the same way we can deliver a certain 


196 


Transformers 


amount of electrical energy at low pressure over 
large conductors or the same amount at high pressure 
over comparatively small conductors. But for 
lighting and many power applications high voltage 
is not only unsatisfactory but dangerous, and here 
is where alternating current through the use of the 
transformer finds its greatest field. The electrical 
energy may be generated at the central station at 
a certain pressure or voltage, and by the use of the 
transformer this voltage may be “stepped up” to a 
much higher value and the energy transmitted over 
small wires to outlying districts or even other cities 
miles distant. At the receiving end it may be 
“stepped down” with transformers to voltages best 
adapted to the purpose for which it is to be used. 
The transformer has no moving parts and requires 
little attention, being frequently installed in isolated 
stations or on poles which are also used to support 
the wires of the transmission line. 

A similar transformation and transmission of 
energy by means of direct-current would require an 
electrical generator driven by an electric motor, 
commonly known as a motor-generator set, to step 
the voltage up at the central station and another 
motor-generator set to step the voltage down at the 
receiving end of the line. These sets would not only 
cost several times as much as the transformers but 
would require constant attention and their reliabil¬ 
ity would be much less than that of the transformer. 
Furthermore, it is a simple matter to build a trans¬ 
former to operate on voltages that are unattainable 
with direct-current apparatus. So aside from the 
fact that first cost, maintenance, and reliability are 
all in favor of alternating-current machinery for 
transmission of power to any appreciable distance, 
in most cases the voltage suitable for the. most 
economical line construction would be unattainable 
with direct-current machinery. 

If a certain amount of water is being delivered 


197 


Electricity and Electrical Apparatus 

through a pipe, say to a water-motor at high pressure, 
a part of that pressure will be used in forcing the 
water through the pipe, and the more water the 
motor takes the more pressure will be used up in 
forcing the water through the pipe. Similarly, when 
electrical energy is being transmitted over a line a 
certain amount of pressure or voltage is used up in 
forcing the current over the wires. This is called 
the line drop and also varies directly as the load or 
the amount of current taken by the receiving end. 
Probably few users of electric lights realize that a 
small variation in their voltage seriously affects 
both the amount of light and the length of the life 
of their lamps. 

As the line drop varies with the changes in load 
so the voltage at the receiving end depends on load. 
If energy was transmitted from the central station 
to outlying districts at the ordinary voltage (110) 
used for commercial lighting, it would be necessary 
to use an enormous amount of copper in order to 
keep this variation of voltage, due to variation in 
load, within satisfactory limits. Here again the 
transformer finds its field. By stepping the voltage 
up at the station and down at the receiving end the 
same energy can be transmitted with a compara¬ 
tively small amount of copper, and a small line drop 
which will be a still smaller percentage drop, due to 
the higher transmission voltage, and with a result¬ 
ing decrease in variation in voltage at the receiving 
end. For instance, suppose it is desired to furnish 
a customer, a mile distant, with energy at 110 volts 
for 40 carbon incandescent lamps with a variation 
due to line drop of 2 volts. This would mean a 
transmission of about 20 amperes, which would mean 
a total line resistance of T V ohm if the transmission 
was made at 110 volts. 

Now let us suppose the transmission was made 
at 2,200 volts. This would mean that a line loss 
of 40 volts would be permissible. Also the current 


198 


Transformers 


necessary to supply the same amount of energy 
would be 2 V of 20 or 1 ampere. For a line drop of 
40 volts a line of 40 ohms resistance would be satis¬ 
factory. As this is 400 times as high as the resistance 
of the line necessary to transmit the energy at 110 
volts, it means that 4^0 of the copper would be 
necessary. In other words, the amount of copper 



Fig. 207. — A General Electric Constant 
Current Transformer. 

necessary to transmit a certain amount of energy 
with a certain percentage line drop varies inversely 
as the square of the voltage. 

As commonly made, the core of a Constant Cur¬ 
rent Transformer is of the shell type, and one of the 
coils is suspended from one end of a rocker arm, 
which has a counterweight on the other end. This 
rocker arm is supported on knife-edge bearings, 


199 























Electricity and Electrical Apparatus 

resting on hardened steel supports, minimizing 
friction. 

The primary coil, commonly the stationary one, 
is connected to the constant potential source of power. 
The secondary coil, usually movable, delivers con- 



Fig. 208. — A General Electric Constant Current 
Transformer, with Case Removed. 

stant current at varying voltage, and the load gen¬ 
erally consists of arc or incandescent lamps, in series. 

Fig. 207 shows the external appearance of a 
constant current transformer as made by the General 
Electric Company, and Fig. 208, in which the case 
has been removed, shows the arrangement of the 
core and coils. 


200 



Transformers 


With the transformer in operation, if a part of 
the load is taken off, or short-circuited, its resistance 
will be decreased and the current in the secondary 
will rise, increasing the repulsion between the two 
coils due to the currents in them. This increased 
repulsion causes the coils to separate more and a 
correspondingly larger leakage of the flux from the 
primary, and the voltage of the secondary is cut down 
proportionally. Consequently the current falls, 
.and if the transformer is properly adjusted, the cur¬ 
rent will return to its former, normal value. 

If more load is put on, the current in the second¬ 
ary is cut down, the repulsion between the coils is 
less and they come closer together. More lines of 
force from the primary interlink with the secondary, 
and the current in it, as a result, rises to its former 
value. 


201 


CHAPTER XXIII 


RECTIFIERS 

Mercury Arc Type — Constant Potential — Constant 
Current. 

It is well known that direct-current arc lamps 
have higher efficiency and better light distribution 
than the alternating current lamps, and with the 
magnetite or metallic flame lamp, for direct-current 
series circuits, these advantages are even more 
pronounced, and also there are additional points in 
its favor, such as a better quality of light and lower 
maintenance cost. 

This is one of the more important instances 
where direct-current is superior to alternating, while 
at the same time, due to their many well-known 
advantages, alternating-current systems of trans¬ 
mission and distribution have been installed in many 
localities. And in these instances, where only alter¬ 
nating-current is available, it is often desirable and 
sometimes necessary to have some means of con¬ 
verting the alternating to direct current. 

Until recently this conversion was accomplished 
by motor generators or rotary converters, both of 
which require large floor space. The former has a 
rather low full-load efficiency and a very low light¬ 
load efficiency, while the latter requires more at¬ 
tention, and should, preferably, be started and ope¬ 
rated only by one familiar with rotary converters. 
Mechanically driven and chemical rectifiers have 
not proven reliable in the past. 

Most of these disadvantages are overcome or 
eliminated when a mercury arc rectifier is employed. 


202 


Rectifiers 


While it has been on the market a comparatively 
short time, the mercury arc rectifier has been in- 



Fig. 209. 



Fig. 210. — General Electric Constant Current 
Rectifier Tube. 

stalled in many cities, and has been in service long 
enough to justify most of the claims of the manu¬ 
facturers. It requires but little floor space and has 


203 











Electricity and Electrical Apparatus 


no rotating parts. The first cost, is low and the 
efficiency at both full and light loads is high. 

Suppose the air is practically exhausted from a 
glass vessel, such as T in Fig. 209, and which contains 
mercury vapor. If a low voltage is impressed on 
the wires WW, leading to the electrodes A and B in 
the tube, no current will flow, as the vapor will act 
as a non-conductor. But if this impressed electro- 



Fig. 211. — Westinghouse Constant Potential 
Rectifier Bulb. 

motive force is increased sufficiently, it will jump 
across, or arc from one electrode to the other, and 
in so doing will break down or ionize the mercury 
vapor so that it will now allow current to flow in one 
direction, but remains practically a non-conductor 
to the passage of current in the opposite direction. 

The operation of the mercury arc rectifier is 
based on this principle or property of mercury vapor. 


204 








Rectifiers 


Fig. 210 represents a tube manufactured by the 
General Electric Company, while Fig. 211 illus¬ 
trates a Westinghouse bulb. Fig. 212 shows a view 
of the complete rectifying outfit, as manufactured 



Fig. 212. — Series Rectifier Outfit for 50-Light System. 
General Electric Co. 

by the General Electric Company, and which con¬ 
sists of a constant-current transformer and re¬ 
actance, an exciting transformer, tube tank and 
tube, static dischargers and switchboard panel. The 
constant-current transformer and reactance coil are 


205 









Electricity and Electrical Apparatus 

contained in one case and air cooled. The tube is 
supported on a wooden holder and immersed in the 
water-cooled oil in the tube tank. The static dis¬ 
chargers are employed to protect the tube and other 
parts of the system from excessive electrical strains. 

The operation of the system and the conversion 
of current can be explained as follows: With ref¬ 
erence to Fig. 213, which is a diagram of con¬ 
nections, suppose we begin to consider the constant- 
current system during a time when the current in 



Fig. 213. — Diagrams of Constant Current and 
Constant Potential Rectifiers. 


the primary of the constant-current transformer is 
flowing from C to D, as indicated by the single-head¬ 
ed arrow. 

If the vapor in the tube is ionized so that it will 
allow current to pass from either anode to the ca¬ 
thode, the section CO of the secondary will cause 
current to flow through the tube from the right-hand 
anode to the cathode, through the ammeter, lamps, 
reactance and back to 0. During this time the 
section OD of the secondary is necessarily idle, as 


206 











Rectifiers 


the vapor in the left-hand part of the tube is a non¬ 
conductor in the direction in which OD would tend 
to make the current flow. (It will be remembered 
that the flow of current in the secondary of a trans¬ 
former is opposite in direction to that of the primary 
during each alternation). 

During the next alternation the current in the 



SaraSBraSSrara 


tPrimary 
(a) mage 


Primary 
(b) Current 


Currentin 

(c) Left Hand 
Anode 

Currentin 

(d) Right hand 
Anode 

Rectified 

(e) Current 


Fig. 214. —Wave Form Without Reactance in the Circuit. 

primary flows from D to C in the direction of the 
double-headed arrow, which symbol also indicates 
the direction of the flow of current throughout the 
circuit controlled by section OD of the secondary, 
OC now being idle. 

In Fig. 214 a and b show the wave form of the 
primary voltage and current, and if the effect of 
reactance coil is ignored the wave form of the 


207 




Electricity and Electrical Apparatus 

current in sections OC and OD of the secondary are 
illustrated by c and d. If these are combined the 
result will be a unidirectional or rectified current, 
as represented by e. 

If such a current as this was used, the rectifier 
arc would go out every time the current became 
zero. Thus there is need of some means to main¬ 
tain or carry over the arc, and this is accomplished 
by the reactance, which is connected between the 



Carnot Ware in 

I 

Positive Electrode.. 


Current Ware in 

n 

Positive Electrode. 


Move of Rectified 

m 

Current 


Ware of Impressed 

nr 

£nr. 


Fig. 215. — Wave Form With Reactance in the Circuit. 

lamps and the point 0 of the secondary of the con¬ 
stant current transformer. 

The effect of the reactance can be likened to a 
flywheel on an engine. That is, the reactance tends 
to keep the current at the same value, and if it starts 
to die away the reactance will tend to sustain or 
keep it flowing. In other words, the reactance 
produces an elongation of the current waves in the 
two anodes, as shown in Fig. 215, I and II. When 
these two are combined they overlap each other and 


20S 





Rectifiers 


the resulting current in the lamps is slightly pulsat¬ 
ing, as represented by Fig. 215, III. 

In starting a tube or bulb for the first time, the 
following precautions should be observed. As a 
preliminary move, with tubes such as shown in 
Fig. 210, it is a good plan to take the tube and with 
the mercury wash out around the anodes carefully, 
being sure that no little dots or particles of mercury 
are left adhering to the glass near the anode tips, 
as if not removed, they may cause the tube to be 
punctured soon after starting up. 

Then place the tube in the holder and make the 
necessary connections. The coils of the constant 
current transformer are pulled apart and the primary 
is “ plugged on” or connected to the source of power, 
the lamps being short-circuited meanwhile. 

The tube holder is next rocked back and forth 
through a small angle, and as this is done, the mer¬ 
cury bridges the space between the exciting anodes 
and the cathode, completing the secondary circuit 
of the exciting transformer and allowing current to 
flow. When the mercury runs back and breaks the 
metallic circuit an arc is formed, ionizing the mercury 
vapor, as explained above, the only difference being 
that instead of using high potential to break down 
the vapor and produce ionization, this is accom¬ 
plished with a low potential, about 110 volts, by 
placing the exciting anodes in close proximity to the 
cathode and by the shaking of the tube, as just ex¬ 
plained. 

It will be noted that when the tube is shaken 
the mercury bridges the space between the exciting 
anodes and the cathode, momentarily short-circuit¬ 
ing the exciting transformer, which is so designed 
that the current under these conditions is not ex¬ 
cessive. The capacity of this transformer is about 
100 watts and it can be disconnected from the cir¬ 
cuit after set is started up. 


209 


CHAPTER XXIV 


A. C. MOTORS AND CONVERTERS 

Induction Motors — Synchronous Motors —Rotary 
Converters. 

Suppose the pole pieces shown in Fig. 216 were 
so supported as to allow their being rotated freely. 
If the coil was stationary the conductors would cut 



Fig. 216. 


lines as the poles passed by, inducing an E.M.F. 
which would cause current to flow in the short- 
circuited coil as indicated in Fig. 217. This current 
would react on the field and produce a force tending 
to cause rotation in the direction pointed out. In 


210 


A.C. Motors and Converters 


other words, the coil would tend to follow the pole 
pieces. However, no matter how easily rotated, 
the coil can never revolve at exactly the same speed 
as the poles, as there would be no lines cut under 


[n 


Fig. 217. 

those conditions, and no induced currents. It must 
necessarily “slip,” or revolve at a slower rate, in 
order to cut lines, induce current, and produce a 
reaction. The greater the difference in speeds, the 




more lines are cut, and within certain limits, the 
greater the turning effort, as the induced currents 
will be larger and the reaction stronger. 

Fig. 218 is a wave diagram of the currents in 
a two-phase circuit. The time, represented by the 


211 





















Electricity and Electrical Apparatus 

horizontal line, is divided into eight equal periods 
by the vertical lines 1, 2, 3, 4, 5, 6, 7 and 8. 

Suppose A, A and B, B (Fig. 219) to be the 
parallel sides of two rectangular coils at right angles 
to each other. If coil A, A is connected to one 
phase and B, B to the other of the two-phase source 
of supply, there will be a peculiar magnetic field 
produced, that will rotate with the relative changes 
of magnitude of current in the two phases, although 
the coils do not move. To trace out the combined 
magnetic effect of the currents in the two coils, 
refer to Figs. 219 to 226, which show the resulting 
magnetic fields when currents have the values and 



directions given in the wave diagram at the vertical 
lines 1, 2, 3, etc. 

Case 1. Current in phase A is zero, current 
in phase B is .a maximum in the positive direction, 
say 10 amperes. The field would be as shown in 
Fig. 219, and a compass needle placed in the centre 
of the two coils would point in the direction shown. 

Case 2 . Current in phase A gradually 
increasing in positive direction, current in phase 
B decreasing. Current in two phases of equal values 
7.07 amperes. It will be noted that the needle has 
rotated to a new position. 

Case 3. Current in phase A has become a 
positive maximum of 10 amperes, current in phase 


212 


A. C. Motors and Converters 

B has become zero. Field produced is due solely 
to the current in the coil A A, and is as shown, with 
the needle pointing along the lines of force. 

Case 4. Current in phase. A gradually de¬ 
creasing, value 7.07 amperes; current in phase B 




gradually increasing in a negative direction, value 
7.07 amperes. 

Case 5. Current in phase A has become zero, 
current in phase B has become a negative maximum, 
10 amperes. 



Fig. 223. — Case 5. 


Fig. 224. — Case 6. 


Case 6. Current in phase A increasing in a 
negative direction, value 7.07 amperes; phase B 
current decreasing, negative value 7.07 amperes. 

Case 7. Current in phase A negative maxi¬ 
mum, 10 amperes; phase B has become zero. 


213 


Electricity and Electrical Apparatus 

Case 8 . Current in phase A decreasing, nega¬ 
tive value 7.07 amperes; current in phase B in¬ 
creasing in the positive direction, value 7.07 am¬ 
peres. 



Fig. 227 — Stator Core of Westinghouse, Type CCL, 
Induction Motor. 

Thus during one cycle the needle has turned 
through 360°. We have produced a magnetism 
which rotates with the changes in current, although 


214 


A. G. Motors and Converters 

the coils were stationary; and the faster the cur¬ 
rents change, or in other words, the higher the 



Fig. 228. — Rotating Field in a Six-Pole 
Induction Motor. 


frequency, the faster the rotation of the compass 
needle. Roughly speaking, this is the idea or 
principle of the induction motor. The synchro¬ 
nous speed of an induction motor is the rate of 
rotation of the field, usually expressed in R.P.M. 

Three-phase currents, with the proper wind¬ 
ings, will produce a revolving field even more 
readily than two-phase, though the action of two 
phases is easier to follow in studying the principle. 
The winding may be either one, two, or three-phase, 
and is placed in the slots of a core built up of lamina¬ 
tions as shown in Fig. 227. 

The coils may be arranged so that each phase 
will produce two, four, six, eight, or even more 
magnetic poles. Fig. 228 shows the magnetic flux 
in a six-pole motor. A winding diagram showing 
the connections of a two-pole single-phase induction 


215 



Electricity and Electrical Apparatus 


motor is given in Fig. 229, while Fig. 230 illustrates 
the connections of a four-pole, three-phase, 24-slot 
winding. 



2-Pole, 16-Slot, 87 %% Pitch. 

Fig. 229. — Winding Diagram for Two-Pole, 
Single-Phase Induction Motor. 


A common form of revolving element, or rotor, 
consisting of a number of bars short-circuited on 
each end by rings, is shown in Fig. 231. On account 
of its construction, this type is known as “ squirrel 



4-Pole, 24-Slot, 83 \% Pitch. 

Fig. 230. — Winding Diagram for Four-Pole, 
Three-Phase Induction Motor. 


cage.” It has no electrical connection with any 
outside source of power, the currents in it being in¬ 
duced by the changing magnetic field, hence the 
name Induction Motor. 


216 

































































































A. C. Motors and Converters 

To a certain extent, an induction motor is a 
special application or adaptation of the principle 
upon which an alternating-current transformer 
operates. For this reason the winding connected 
with the source of power is often called the primary, 
and the element in which the current is induced, is 
called the secondary. In ordinary commercial 
motors it is customary to have the primary winding 
placed in the stator, or stationary frame of the 
machine. As this is the only winding connected 



Fig. 231. — Squirrel-Cage Rotor Winding. 


to a source of power, this simplifies the machine and 
avoids sliding contacts and brushes. 

For variable speed work, or when a particularly 
large turning effort or “torque’ is desired in start¬ 
ing, such as on crane work, wound rotors are some¬ 
times employed and external resistances are con¬ 
nected in series with these windings by means of 
slip rings and brushes. The reason for this will be 
apparent from a consideration of the following 
paragraph. 

If the bars of a squirrel-cage rotor are large 
and of low resistance, large currents are produced 
in them even though they cut a comparatively small 


217 


Electricity and Electrical Apparatus 

number of lines of force; consequently such a rotor 
will run with very little slip, and its speed will be 
reduced very little by an increase in the load it 
must drive. On the other hand, such a rotor has 
the peculiar effect of reducing the torque of the 
motor when starting up or running at speeds greatly 
below synchronism, although the current flowing 
becomes very large. 

The induction motor is adapted to many uses 
that would be impossible for other motors, from 



Fig. 232. — General Electric, Form K, 
Squirrel-Cage Rotor. 


the fact that no brushes or slip rings or commutators 
or other bare or moving parts carrying current are 
required (currents in the squirrel-cage rotor being 
of very low voltage and flowing entirely in itself, 
much like eddy currents). All the parts that are 
connected to the supply wires are stationary and 
may be covered with heavy insulation. Fig. 233 
shows an induction motor, built by the Lincoln 
Electric Company, in which the current carrying 
parts are so well insulated and other parts are so 
designed that it will operate even under water. 


218 





A. C. Motors and Converters 

We have seen that an alternating current in 
the stator of an induction motor, produces a re¬ 
volving field. We have also seen in Chapter XXI, 
that a revolving field produces an alternating cur¬ 
rent in a stationary armature winding. Now an 
alternator armature winding is, in principle, the 
same as the stator winding for an induction motor 
of the same number of poles and phases. 

Let us remove the rotor from an induction 
motor, and substitute a revolving field structure 
similar to that in Fig. 117, with the field magnets 



Fig. 233. — Type I, Induction Motor. 

Lincoln Electric Co. 

excited. We will also cause this field to revolve at 
the same speed and in the same direction as that of 
the stator. If we keep each north pole of this new 
rotor opposite a south pole in the stator field, and 
vice versa , there will be an attraction between them. 
Now a little reflection will show that if either field 
tends to run slower, this attraction will speed it up 
and keep it opposite the other. If the rotor field 
is driven a little ahead of the stator field, current is 
generated in the stator windings and sent out on 
the line, and we have a regular alternating current 


219 


Electricity and Electrical Apparatus 

generator. If the rotor is not driven but is con¬ 
nected to a load so that it lags a trifle behind the 
stator field, the latter exerts a pull, and keeps it 
running as a motor. In this case, energy is supplied 
'to the stator winding from the line, to maintain the 
pull on the rotor. 

An alternator, then, will run as a motor if 
current is supplied to it. It makes no difference 
whether the armature or the field-magnets revolve, 
the effect will be the same. A machine built to be 
driven in this way by alternating current is called 
a Synchronous Motor, because it must always run 
at synchronous speed. This speed it will be seen 
is exactly that of the generator which supplies the 
current if generator and motor have the same num¬ 
ber of poles. 

If we recall now the discussion of the direct- 
current dynamo in Chapter X, we will remember 
that the current flowing in each coil is alternating 
and that a commutator is necessary to rectify it 
into direct current. We are now prepared to notice 
that each coil on a direct-current armature gene¬ 
rates an alternating electro-motive force that is 
different in phase from that generated in every other 
coil on the armature. If, however, taking those coils 
in which the phases are most nearly alike for one 
phase, we divide the winding up into sections corre¬ 
sponding to the number of phases required, we can 
put a set of slip rings on the armature at the other end 
from the commutator, connect them to the points of 
division, and take direct current from one end and 
alternating from the other end of the same armature. 

We should notice, however, that if the machine 
is adjusted to furnish direct current at a certain 
voltage, it will give a certain voltage at the alterna¬ 
ting current end. Changing the -voltage of either 
will change the other. 

Such a machine, then, will furnish both direct 
and alternating current, or it will furnish either 


220 


A. C. Motors and Converters 

alone. It will run as a motor if supplied with either 
kind of current — as a synchronous motor if alter¬ 
nating current is used. Also — and this is the im¬ 
portant point — we can supply either kind of cur¬ 
rent to it, and obtain the other kind from it at the 
other end of the armature. When used in this 
way the machine is known as a Rotary Converter. 

Rotary Converters are most largely used in 
electric railway work, where direct current of about 
600 volts must be supplied to the trolley wires, and 
it is desired to transmit the power from a distance. 
It would require an enormous amount of copper to 
transmit the large currents used for any consider¬ 
able distance, if no higher voltage were used. But 
by using alternating current and high voltage, the 
current, and therefore the wires, are small, and we 
can “step down” the voltage by means of trans¬ 
formers near the point where it is to be used, and 
then change it to the required direct current by 
means of rotary converters. 

Synchronous motors must be brought up to 
speed and “synchronized” — that is, adjusted so 
that motor and line are in phase with each other —■* 
before connecting it to the line. Several methods 
of doing this are used. A small induction motor 
may be connected to its shaft, arranged to start it 
and bring it up to a speed somewhat faster than 
synchronism. Current is then cut off and it is 
allowed to “drift” down to the proper speed, and 
when exactly in phase, as shown by special synchro¬ 
nizing devices, it is switched onto the line. 

A second, and more common method, is to 
supply the armature of the rotary converter or 
synchronous motor with alternating current at a 
voltage considerably lower than that of the line, 
obtained by means of low voltage taps from the 
transformers, or by starting compensators. The 
field is not excited until the machine has nearly 
reached synchronous speed. The armature will 


221 


Electricity and Electrical Apparatus 


act on the solid pole faces as on the bars of a squirrel- 
cage rotor, bringing it up to speed as an induction 
motor. Then the field-current is applied, locking 
the machine in step with the supply current. 

Railway converters are sometimes started with 
a starting-box from the D.C. side, if there is direct 
current being supplied to the trolley at the time by 
another converter. 


222 


CHAPTER XXV 


MOTOR CHARACTERISTICS 

Characteristics of Shunt, Series,and Compound-Wound 
Direct-Current Motors — Characteristics of Alter¬ 
nating-Current Motors. 

Generator characteristics have been explained 
in Chapter XX. Motors have more characteristic 
curves than generators, since there are more quanti¬ 



ties of interest to the designer or purchaser which 
will change with the load. The more important of 
these variables are speed, horsepower, torque, and 
efficiency. 


223 
























Electricity and Electrical Apparatus 



Fig. 235. — Characteristic Curves of Westinghouse No. 93 A Series 
Railway Motor, Geared to Car Axle. Gear Ratio, 16 to 71. 
Wheels, 33" Diameter. 















































































































































































































Motor Characteristics 


The torque depends not only on the armature 
current, but on the strength of field as well. For 
this reason, where strong turning efforts are re¬ 
quired, -especially at starting, series field windings 
are used so that with a heavy load .not only the 
armature current but also the field is strengthened. 
In shunt field motors, extra torque is gained only 
by added armature current, the field strength being 
practically fixed. To better understand each ma¬ 
chine, let us consider their characteristics separately. 

Fig. 234 shows characteristic curves of a 10-H.P., 
230-volt, 1000-R.P.M. series motor. In this diagram 
the horsepower developed by the motor is" plotted 
as abscissas and used as the basis on which other 
data are shown. Values for the ordinates of speed 
and efficiency curves are given on the left, those for 
torque and motor current on the right.* 

The curve of R.P.M. shows that while the speed 
falls off on heavy loads, on light loads it runs up to 
very large values. This curve gives 980 R.P.M. at 
full load, 1350. at half load, and 505 at double load. 
If the load were entirely disconnected, the speed 
would become excessive, causing armature and 
commutator to fly to pieces, because of centrifugal 
strains. It is, therefore, always necessary to 
direct-connect a series motor to its load. Belts 
should never be used, as they are liable to break 
or fly off on overloads. 

Note that at rated H.P. the torque is 55 lb.-ft., 
while at 20 H.P. it is 200 lb.-ft., nearly four times 
the torque at double load, whereas a shunt machine 
would show only a little over double torque on 
double load. 

* Values of torque are given in pound-feet, or effective 
pull in pounds at one-foot radius. For example, if an 
armature has 200 conductors, and each conductor, at a dis¬ 
tance of 10 inches from centre of shaft, has an average pull 
of 2.5 pounds, the torque or twisting effort would be 200 X 
2.5 X 10 = 5,000 pound-inches, or 5,000/12 = 416 pound-feet. 


225 


Electricity and Electrical Apparatus 

This diagram also shows the efficiency curve or 
per cent, useful energy obtained from motor at 
different loads. The best efficiency on this partic¬ 
ular motor is 89% at full load. The efficiencies 
at half load, 83.5%, and double load, 81%, show it 
to be well designed. Currents for different horse¬ 
power outputs are shown in motor-current curve. 
Motors requiring maximum torques and where 
change of speed with load is not objectionable are 
series wound. Railway motors require large torque 
to get a car or train under way, while as car speeds 



Fig. 236. — Characteristic Curves of a Shunt Motor. 

up less torque is required. These requirements are 
ideally fulfilled by the series motor. So with other 
applications discussed in next chapter.* 

♦Fig. 235 shows curves for a No. 93A Westinghouse 
Railway Motor. These curves present many interesting 
facts concerning railway motors. The amperes input are 
given as abscissas, and all other values are plotted with 
reference to amperes input. These curves show* not only 
speed, efficiency, horse-power, but the speed of car, and 
tractive effort or pull exerted at the rails, with 33-inch 
wheels and gear ratio 16 to 71, and also the time motor can 
be run under given loads to obtain specified temperature 
rises. This motor on a one-hour rating to give 75° C. rise 
shows 60 Brake H.P. 


226 






















Motor Characteristics 


Fig. 236 shows characteristic curves of a 40-H.P., 
500-volt, 500-R.P.M. shunt motor. These curves 
are similar to Fig. 234, except that speed varies 
little with load and torque curve is much less steep. 
Note that the torque goes up approximately with 
the load, and not rapidly as in a series motor. Also 
that the efficiency is a little higher than that of the 
10-H.P. motor in Fig. 234. Larger motors have 
less percentage losses and therefore show higher 



Fig. 237. — Characteristic Curves of Shunt Motor 
With and Without Commutating Poles. 


efficiencies. For motors where constant speed at 
all loads is the essential, shunt windings are used. 
Shunt motors can be run with perfect safety at no 
load.. 

The addition of commutating poles to direct- 
current machines, as explained in Chapter XIX, 
not only increases the efficiency by reducing arma¬ 
ture short-circuit currents but also affords better 


227 

































Electricity and Electrical Apparatus 



Fig. 238. — Characteristic Curves of a Westinghouse Inter-Pole 
Railway Motor. 


228 

























































































































































































































































































































































































































































Motor Characteristics 

speed regulation, that is, less change of speed from 
no load to full load. Fig. 237 shows curves of a 
30-H.P., 230-volt motor with and without com¬ 
mutating poles; full-line curves with, and dotted 
without.. Without these poles the speed goes from 
625 R.P.M., at no load, to 600 R.P.M. at full load, 
a change of 25 R.P.M., or 4%, while the commuta¬ 
ting poles show no load 632 R.P.M. and 623 R.P.M. 



Fig. 239. — Westinghouse No. 305, 75 H.P., 600 V., Inter- 
Pole Railway Motor. Lower Frame Down for 
Inspection. 


at full load, a difference of 9 R.P.M., or 1.5%. As 
commutating-pole machines are lighter and cheaper, 
they are coming into universal use, not only for 
stationary motors, but for series railway motors. 

Compound motors are a combination of series 
and shunt. Generally they are series motors with 
sufficient shunt field to prevent motor racing on no 


229 




Electricity and Electrical Apparatus 

load, and curves are similar to Fig. 234, except speed 
and torque curves are less steep. 

Characteristic curves for motors are obtained 
in two ways: First, by calculations from dimen¬ 
sions and electrical specifications. This method 
is accurate though very laborious, requiring many 
hours of slide-rule work. The experienced designer 
can plot curves showing every action of a new 
design of machine before even the patterns for 
castings or dies ‘for punchings have been started. 

Another and more rapid, though less accurate 
method, is to determine data from test and then 
plot the results. The method of obtaining these 
results is by the use of a “Prony Brake.” 



Fig. 240 shows a Prony Brake. This con¬ 
sists of an arm made of wood or steel, depending on 
size, and a strap to put over pulley with turn buckle 
as shown. The strap is often fitted with small blocks 
of wood to take the wear, and pulleys are made 
special, with flat instead of crown face, and with 
side flanges on the inside of rim to hold water when 
in motion. The water boils and carries off the heat 
which otherwise would soon burn the wood blocks. 
The arm is generally balanced by a weight, as 
shown, so that when pulley is not revolving the 
scales show no reading. If arm is not balanced, a 
“tare” reading must be noted and subtracted from 
all scale readings. 


230 











Motor Characteristics 


Knowing the horizontal distance between center 
of shaft and point of support on scales, and the scale 
reading, the torque at any speed and current input 
can be determined. For example, suppose R is 
3 feet, the scales give 17.6 lbs., current is read 29.1 
amps, at 230 volts, and speed is noted to be 796. 
The torque may then be calculated as 

T = 17.6 X3 = 52.8 pound-feet. 



Fig. 241. —Connections for Brake Test. 
Direct Current. Shunt Motor. 


There are several simple formulae used to 
calculate data from a test of this kind, as follows: 


Torque = T 


H. P. Output 


= Length of Arm X Net Weight 
= RX F 

2 X 3.14 X R.P.M. X RX F 
33,000 

R.P.M. X T 
5250 


Watts Output = 746 X H.P. Output 


Watts Input 
Efficiency 


= .142 X R.P.M. X T 
= Line Volts X Motor Current 
_ Watts Output 
Watts Input 


A sample test on a 7.5-H.P., 800-R.P.M., 230- 
volt shunt motor with a 3-foot brake arm is given. 
Field current was .80 amperes during test. Volt- 


231 

















Electricity and Electrical Apparatus 


age was held constant at 230. Tension on strap 
was varied from approximately 4 to 36 pounds. 
Speeds, currents and actual scale readings were 
noted at the same instants. 


Readings. 

Calculations. 

Armature 

Current 

R.P.M. 

Net 

Weight 

Motor 

Current 

o 

!h 

o - 
H 

Watts 

Output 

W atts 

Input 

Effic. 

% 

H.P. 

Output 

8.8 

830 

4.2 

9.6 

12.6 

1490 

2210 

67.5 

1.99 

15.1 

815 

8.6 

15.9 

25.8 

2990 

3660 

81.7 

4.00 

21.9 

805 

13.0 

22.7 

39.0 

4460 

5230 

85.4 

5.98 

29.1 

796 

17.6 

29.9 

52.8 

5970 

6880 

86.8 

8.00 

37.0 

788 

22.2 

37.8 

66.6 

7460 

8700 

85.8 

10.0 

45.0 

783 

26.8 

45.8 

80.4 

8940 

10500 

85.0 

12.0 

54.0 

777 

31.5 

54.8 

94.5 

10450 

12600 

83.0 

14.0 

63.5 

773 

36.3 

64.3 

109.0 

12000 

14800 1 

1 

81.1 

16.0 

Field Current, .8 Amperes. 

Brake Arm, 3 Feet. 




232 














































Motor Characteristics 


Fig. 241 is a diagram of the electrical con¬ 
nections for this test. Fig. 242 shows a plot of these 
values. It is always desirable to repeat a test and 
plot average values. Tests should be run when 
motor is hot, that is, after having been run for 
several hours, otherwise the rise in temperature in 
fields causes an increase in resistance, decrease in 
current, and consequent increase in speed during 
the test, which is undesirable. 



Fig. 243. — Characteristic Curves of Three-Phase 
Induction Motor. 7^-H.P., 4-Pole, 60-Cycle, 1800 
R.P.M. Synchronous Speed. 


Water brakes are used on high speeds, 3000 
R.P.M. and above, also on large powers. The 
water brake consists of a disc or series of discs run¬ 
ning in water. The housing is balanced and through 
an arm transmits the twist to scales which measure 
twist in pounds, the water acting as a medium 
instead of the belt in the brake shown in Fig. 240. 
The water is piped so as to flow continuously through 
the brake when desired. 


233 































































































Electricity and Electrical Apparatus 

Prony brakes can be used to measure the power 
of any engine or shaft as well as a motor by a modi¬ 
fication of design. A convenient method of measur¬ 
ing the power required by machine tools is as follows: 
Carefully test a motor and plot curves. Then drive 
the machine tool by this motor and note current. 
With this value of current, read off from curve the 
horsepower to drive the machine in question. 

The characteristic curves of induction motors 
are very similar to the direct-current curves, and 
can likewise be either calculated or determined from 
a brake test. Fig. 243 shows curves of a 7.5-H.P., 
1800-R.P.M., 220-volt polyphase motor. Horse- 



Fig. 244. — Connections for Brake Test. 
Single-Phase Motor. 


power output is used as a base line, and efficiency, 
power factor, amperes input and slip are plotted, 
showing their several relations to the horsepower 
output. 

Unlike direct current, the product of the read¬ 
ings of ammeter and voltmeter does not show the 
real power consumed, but a value called “apparent 
power.” The wattmeter shows the real power. 
The real power divided by the apparent power is 
called the power factor. This value can be at once 
calculated from instrument readings. 

Fig. 244 is the diagram of a single-phase motor 
connected for brake test, while Figs. 245 and 246 * 


234 
















Motor Characteristics 


show connections for three and two-phase motors. 
For the power input of a polyphase motor, always 
take the algebraic sum of the wattmeter readings. 



Fig. 245. —- Connections for Brake Test. 
Three-Phase Motor. 


To get the watts expended in each phase, divide 
this sum by the number of phases. Except in very 
special tests, power losses in the measuring instru- 



Fig. 246. — Connections for Brake Test. 
Two-Phase Motor. 


ments are neglected. Values for efficiency curve 
are then calculated as for direct-current machines. 

The current-input curve represents current per 
phase in the case of a polyphase motor, and is taken 
as the average of ammeter readings. 


235 






































































Electricity and Electrical Apparatus 

As has been noted in a preceding chapter, an 
induction motor cannot run at synchronous speed, 
but must have a certain amount of slip to provide 
the torque necessary to carry the load. Now the 
synchronous speed is dependent on the frequency 
and number of poles, as follows: 


Synchronous Speed = 


Frequency X 120 
Number of Poles 


The actual speed of the motor is less than this 
amount, and the slip is expressed as a per cent, of 
synchronous speed, as follows: 

_ Synchronous — Actual Speed 
p Synchronous Speed 



Fig. 247. — Speed-Torque Curve of an Induction Motor. 

A very interesting curve, and one which shows 
at a glance some of the more important characteris¬ 
tics of an induction motor, is the speed-torque curve, 
Fig. 247. Curve I is for a motor with low-resistance 
rotor winding. The effect of this winding in re¬ 
ducing the starting torque is plainly seen, as well 
as its effect at all speeds. The maximum torque 


236 


































Motor Characteristics 


is developed at a speed of about 85% of synchronous 
speed, then falls off rapidly as the motor reaches its 
running speed, which is found by locating the point 
on the curve at which the required torque is de¬ 
veloped. This part of the curve is very steep, 
showing small changes in speed for large changes in 
load. If the motor is driven so as to run faster than 
synchronous speed, the torque is negative. If back¬ 
wards, the opposing torque becomes less and less 
as the speed increases. 

Curve II shows the effect of inserting resistance 
in series with the rotor. The speed at full load is 
less, i.e., the slip is greater, than in the case of Curve 
I, the speed of maximum torque is less than before 
and the torque at starting is very much higher. It 
is readily seen that the maximum torque can be 
located at any speed desired, by inserting the re¬ 
quired amount of resistance. This is the reason 
wound rotors with external resistance are used for 
elevator and crane motors. 

The following formulae are for use in alternating- 
current calculations. They will be taken up and 
fully explained, together with other characteristics 
of alternating current apparatus, in Vol. II of this 
text book. 

To calculate the watts power when current per 
phase, volts, and power factor are known: 

Single-Phase, P = E X / X P.F. 

Two-Phase, P = 2 X E X / X P.F. 

Three-Phase, P = V3 X E X / X P.F. 

= 1.732 X E X / X P.F. 

To calculate line current per phase when the 
power in watts, voltage, and power factor are known: 

P 

Single-Phase, I = g p F 


237 



Electricity and Electrical Apparatus 


Two-Phase, 


I = 


P 

2 X E X P.F. 


Three-Phase, I = 


P 

Vs x E X P.F. 


To calculate line current per phase when volt¬ 
age, power factor and horsepower of the motor are 
known: 


Single-Phase, 


H.P. X 746 
Eff. X E X P.F. 


Two-Phase, 


H.P. X 746 
Eff. X 2 X E X P.F. 


Three-Phase, 


H.P. X 746 

Eff. X Vs X E X P.F. 


Direct-Current, / = 


H.P. X 746 
Eff. X E 


In thesfe formulae: 

P = Power, in Watts 
E = Electromotive Force in Volts 
I = Current in Amperes 
Eff. = Efficiency 
P.F. ■= Power Factor 
H.P. — Horsepower 


238 








CHAPTER XXVI 


MOTOR DRIVE 

Transmission of Power — Mechanical — Electrical — 
Efficiency of Transmission — Adjustable Speed 
Motors — Applications to Which Each Kind of 
Motor is Especially Suited — Power Required 
for Machines. 

The transmission of power mechanically from 
a source of generation to the place of utilization 



Fig. 248. — Corliss Engine Belted to Line Shaft. 

requires shafting, belting, rope drive, or often gears. 
We are familiar with the factory in which one or 


239 




Electricity and Electrical Apparatus 


more slow-speed reciprocating engines, drive a heavy 
line shaft from which belts extend to other shafts 
and countershafts throughout the plant, with noise, 
dirt, and confusion, poor light and restricted vision. 

The change wrought when electric is substi¬ 
tuted for mechanical transmission in such a factory, 
is a revelation to one not acquainted with the 
benefits derived from motor drives. Belts and 



Fig. 249. — Direct-Connected Generating Set. ? 
Ridgway Dynamo and Engine Co. 


shafts give way to clear space, free air and head 
room, light is unobstructed, and the walls and ceil¬ 
ings, which moving belts blacken and give a smoky 
appearance, can be kept white and clean. Noise is 
limited to the machines themselves, and to those 
alone which are in actual operation, and the factory 
takes on a general appearance of order and arrange¬ 
ment. And these are not the only, nor, indeed, in 


240 





Motor {..Drive 


the eyes of many managers, the most important 
considerations. 

Besides involving a high first-cost for equip¬ 
ment, the mechanical distribution of power is very 
inefficient. Usually 20 to 40% and in some cases 
as high as 60 or 75% of the power developed by the 
engines or water wheels is lost in transmission and 
never reaches the machines for which it was in- 



(By permission of the General Electric Co.) 

Fig. 250. — Group of Lathes, Belt Driven. 

tended. For example, a 250-H.P. engine run at 
full load loses 30% or 75 H.P., in bearings, belts, 
etc. In a year the power thus wasted will represent 
a large sum. 

Sometimes clutches are arranged to cut out 
parts of the shafting so that any particular part of 
the factory may be run alone. These, however, are 
seldom installed, and usually it is necessary to run 


241 






Electricity and Electrical Apparatus 

the entire system when one part of the factory, or 
even a single machine, is to be operated overtime. 
Thus if only 25 H.P. were required to operate one 
room at night, the engine mentioned above would 
have to deliver nearly 100 H.P. Also, besides 
inefficiency, the maintenance cost of such a system 
is high, requiring constant attention, tightening 



(By permission of the General Electric Co.) 

Fig. 251. — Group of Lathes, Motor Driven. 

belts, oiling and rebabbiting bearings, which are 
often in awkward and inaccessible places. 

The modern method of motor drive makes it 
possible to replace the large slow-speed engines by 
lighter and smaller high-speed engines or turbines, 
requiring much less floor space and head room — 
often an important consideration in city installa¬ 
tions. With electric-motor drive, the engine and 


242 









Motor Drive 


generator can be placed in any part of the factory, 
regardless of line shafting. Wires or cables carry 
the power to the switchboard, and thence to the 
several departments of the factory where power is 
used, through walls or floors, around corners or 
angles — anywhere out of the way — wherever wire 



Fig. 252. — Group of Punch Presses, Operated by 
General Electric Induction Motors. 

Note that presses are arranged for economy of floor space in 
handling stock and scrap. This grouping would be impracticable if 
presses were driven from line shafting. 

can be strung. Corners or angles that would cause 
serious losses in belting or gearing in no way affect 
the electric transmission line. 

Let us consider some of the ways in which the 
power consumption is reduced and power bills cut 
down by motor drives. 


243 












Electricity and Electrical Apparatus 

In the first place, power losses in transmission¬ 
line wires do not often exceed 5% and are often 
lower than this. Motor efficiencies range from 80% 
in small sizes or at light loads, up to 90 or 92% in 
larger capacities. Thus we see when all machines 
are running, the losses are only 10 to 25 %, as opposed 



Fig. 253. — Westinghouse Polyphase Motor, Driving 
Swing Saw. 

to the heavy losses incurred in mechanical trans¬ 
mission. 

Again, in the old-fashioned factory, when any 
machine is shut down, the engine is relieved of only 
the actual power taken by the machine. The losses 


244 














Motor Drive 


incident to getting power to the machine go right 
on. In the modern factory, when a machine stops, 
not only the power used by the machine stops, but 



Fig. 254. — Wagner Polyphase Induction Motor, 
Operating a Planer. 

also the losses in the transmission line. While in 
any case, wire losses are a very small consideration, 


245 







Electricity and Electrical Apparatus 

yet it may be worth while to note that these losses 
are reduced, not in proportion to the power, but in 
proportion to the square of the power. For instance, 
three machines connected to one line are taking 
1000 watts each, with a loss of 90 watts or 3% in 
the line. One machine is shut down. Now the 
wire loss is 3000 2 : 2000 2 = 90 : X, whence 



watts. 



Fig. 255. — Wagner Polyphase Induction Motor, 
Operating Endless Pan Conveyor. 

This is 2% of the power, 2000 watts, instead of 3% 
or 60 watts, as it would be if the loss were directly 
proportional to the power. 

Another way in which power consumption is 
reduced has its basis in the ease with which electric 
power is measured. The engineer formerly had 
no way of telling how much load his engine was 
carrying, except in a general way, from its sound 


246 








Motor Drive 


and behavior, or by tedious calculations from indi¬ 
cator cards. Even then he could not tell what part 
of the factory was throwing an excessive load on the 
engine. With electrical equipment, the engineer 
can tell by a glance at the switchboard measuring- 
instruments just what the total load at the time is, 
how much is used in each part of the factory, or 
whether the load is constant or fluctuating. If 



Fig. 256. — Westinghouse Polyphase Induction Motor, 
Geared to Portable Lathe. 

any part of the factory is taking more power than 
usual, it is an easy matter to follow it down to some 
particular machine, and perhaps locate and remedy 
a defect that would have consumed a great deal of 
power before being noticed in an old-time factory. 

Before deciding on the kind of power or type 
of motors to be adopted, the characteristics of the 
various types should be studied with special refer¬ 
ence to the case in hand. There is an electric motor 
adapted to almost every power requirement. 


247 





Electricity and Electrical Apparatus 


Motors are frequently built into the machines 
they are designed to operate, so as to become a part 
of the structure. If specially constructed, motors 
can be operated under water. By the use of enclos¬ 
ing covers they can be operated in powder mills, saw 
mills, cotton mills, or other places where sparks 
would be destructive. Great advances have already 



Fig. 257. — Westinghouse Electric Locomotive. 

been made in the field of steam railroads, where 
electric locomotives are replacing steam. On battle¬ 
ships, almost every motion is accomplished by motor- 
drive. Guns are raised or lowered, their breeches 
opened and closed, ammunition hoisted and turrets 
revolved by specially constructed motors; doors 
connecting the water-tight compartments are closed 
by powerful motors; anchors are drawn, ventilating 
and exhaust fans and pumps operated. 

For most purposes where a constant speed is 
desired, the induction motor is far superior to direct- 
current machines in general reliability and lower 
cost of maintenance. Wherever possible, and if 


248 







Motor Drive 


there are no other considerations which take pre¬ 
cedence over these, the induction motor should be 
used. 

In some cases where the speed must be exactly 
the same under varying loads, synchronous motors 
are used. However, synchronous motors are prone 
to trouble, both in starting and operation, unless 
handled by one familiar with them. 

If adjustable speed is required, the direct-cur- 
rent motor is superior. Adjustable-speed induction 
motors are expensive and cumbersome. Speed re- 



Fig. 258. — Wound Rotor, General Electric, Type M. 

duction is accomplished by introducing resistance 
in the circuit of the rotor, which has, in this case, 
a regular armature winding provided with slip rings 
which connect with a variable external resistance. 
Fig. 258 shows the General Electric type M wound 
rotor with collector rings. 

To understand the effect of this resistance, refer 
to the discussion of speed-torque curves, Fig. 247, 
in the previous chapter. By adjusting the resist¬ 
ance to the required value, the torque necessary 
to carry the load can be developed by the motor at 
any desired speed. It is apparent that the speed 
is very unstable with large resistance in circuit, as 
slight variations in load will produce great changes 
in speed. Also, under load, a great deal of energy 
is wasted in the resistance. Induction motors are 
sometimes so wound that the number of poles may 
be changed by means of a controller which makes 


249 


Electricity and Electrical Apparatus 


different connections. This, however, gives a very 
limited number of speeds with large steps between. 

Coming now to the consideration of direct- 
current motor characteristics, we will take up the 
three general classes: series, shunt and compound. 



Commutating poles are adapted to all three, in¬ 
creasing their stability and fitness for commercial 
demands. 









Motor Drive 


Each type has its own field of usefulness — 
series motors where large starting torque is required 
and speed variation with changes in load is not a 




detriment, shunt motors where change in load must 
not cause appreciable change in speed, and com- 


251 








Electricity and Electrical Apparatus 


pound motors for special cases, where the essential 
characteristics of shunt and series motors must be 
combined. 




Fig. 262 . — Sprague Round Type Motor, Operating Job 
Press by Friction Drive. 


252 




Motor Drive 


Series motors are not at all suited to many uses 
admirably adapted to shunt motors, or vice versa. 
To illustrate: A shunt winding on a railway motor 
would mean abnormally large motors and heavy 
currents to get necessary starting torques. Again, 
on a machine tool, such as a boring mill, a series- 
wound or heavily-compounded motor would cause 
disastrous results. On boring out a field frame, 
for example, the tool would cut during only part of 
the revolution, as from A to B and C to D in Fig. 263. 
When cutting, the motor w r ould be loaded and run 
at a certain adjusted speed. As the tool leaves the 



Fig. 263. — Boring Out a Field Frame. 

pole face, the motor would speed up and the tool 
would hit the other face at high speed and either 
break or dig into the metal. 

For variable speed work, the direct-current 
shunt-wound motor stands supreme. When speci¬ 
ally designed for the purpose, with commutating 
poles, it is both light and cheap. This motor gives 
a smooth scale" of speeds — not long steps from 
lowest to highest. 

Speed may be adjusted in two ways: First, 
the motor may be slowed down by cutting resistance 
into the armature circuit. This, however, is very 


253 


Electricity and Electrical Apparatus 

wasteful, as for instance if we wish to run the motor 
at half speed we must use up half the voltage in the 
resistance. Also the resistance must be heavy and 
cumbersome, as it carries the full armature current. 
Secondly, the speed is increased by weakening the 
field. Resistance is cut into the field circuit. No 
energy is lost, as the current in the field-windings 





Fig. 264. — Boring Mill, Operated by General Electric 
Variable Speed Shunt Motor. 


represents a dead loss of about 2% or 3% in any case, 
and as this current is reduced, the per cent, is still 
smaller. Sometimes a combination of the two 
methods is used, but in such cases the speeds brought 
under armature control should be as few as possible. 

There are also one or two notable instances of 
satisfactory mechanical means for obtaining speed 


254 



Motor Drive 



Fig. 265. - Reliance Adjustable Speed Motor. 


Fig. 266. — Shaper, Driven by Reliance Adjustable 
Speed Motor. 



255 



Electricity and Electrical Apparatus 

variation, such as in the motors made by the Re¬ 
liance Engineering Works. There, the armature is 
conical in shape and is pushed in and out by turn¬ 
ing a hand-wheel. The air gap is varied accord¬ 
ingly, changing the reluctance of magnetic circuit 
and effective strength of field. 



Fig. 267. — Drill Press, Driven by Reliance Adjustable 
Speed Motor. Speed Control Convenient to 
Operator. 

With the introduction of motor drive into 
almost every industry, frequently a middle course 
is pursued by dividing the factory into sections, 
each section being driven by one large motor. This 
method, although not entirely eliminating over¬ 
head shafts and belts, combines many of the im- 


256 


Motor Drive 


portant advantages of individual-motor drive with 
lowered first cost — the one motor replacing several 
smaller ones. 



Fig. 268. — Bolt Cutter, Driven by Reliance Adjustable 
Speed Motor. 


Below is given a list of the more important 
applications, with the type of motor best adapted 
for each use. 

Direct-Current Series Motors. 


Electric Locomotive 
Street Railway 
Crane 

Hoist and Derrick 
Battleship, Elevating 
Guns and Turning 
Turrets, Hoists, Com¬ 
partment Doors, etc. 
Automobile 
Air Compressor 
Blower and Exhaust Fan 


Coal Pick 
Turn Table 
Lifting Bridge 
Rock Breaker 
Gate Valve 
Ceiling Fan 
Launch 

Church Organ Blower 
Vacuum Cleaner 
Forge Blower 


257 



Electricity and Electrical Apparatus 




Fig. 270. - Pipe-Threading Machine, Geared to Crocker- 
Wheeler Compound Motor. 


258 





Motor Drive 


Direct-Current 

Power Shear 
Motor Generator Set 
Balancer Set 
Punch Press 
Drop Forge 
Tumbling Barrel 
Concrete Mixer 
Bread Mixer 
Elevator 


Compound Motors.* 

Conveyor 
Portable Drill 
Ore Crusher 
Pipe Threader 
Rolling Mill 
Printing Press 
Gate Valve 
Sewing Machine 



Fig. 271. — Ice Cream Freezer, Geared to Crocker- 
Wheeler Shunt Motor. 



Fig. 272. — General Electric Portable Breast Drill. 

* Compound field about 20% to 25% of total field, 
except in special cases. 


259 


Electricity and Electrical Apparatus 


Direct-Current Shunt Motors. 


Lathe 

Bending Machine 

Drill Press 

Brick Machine 

Slotter . 

Oil Press 

Shaper 

Lawn Mower 

Boring Mill 

Can Making Machine 

Milling Machine 

Track Drill 

Rivetting Machine 

Bottle Washer 

Gear Cutter 

Bottle Corker 

Moulding Machine 

Bottle Labeler 

Planer 

Grinder 

Laundry Machinery 

Buffer 

Oil Switch 

Coffee Grinder 

Band Saw 

Beef Cutter 

Buzz Saw 

Cloth Cutter 

Jig Saw 

Centrifugal Pump 

Swing Saw 

Cotton Mill Machinery 

Automatic Stoker 

Flour Mill Machinery 

Cold Steel Saw 

Feed Pump 

Ice Cream Freezer 

Tool Grinder 

Exhaust Fan, Adjust¬ 

Farm Machinery 

able Speed 

Pianola 

Shoe Machinery 

Electric Clippers 

Leather Machinery 

Abbatoir Apparatus 

Printing Press 

Dentist Drill 

Linotype 

Massage Vibrator 

Folding Machine 


Induction Motor, 

Special Design with Armature Control. 

Electric Locomotive Hoist 

Crane Air Compressor 

Induction Motor, 

Squirrel-Cage Type. 

For all cases listed under direct-current shunt 
motors, except where adjustable speeds are desired; 
and, specially designed to give added torque, for 
those listed under compound motors. 


260 


Motor Drive 



Fig. 273. — General Electric Variable Speed Type CR 
Motor, Geared to Newton Slotter. 



Fig. 274. — Sprague Round Type Motor, Mounted in 
Pedestal of Saw Table. 


261 





Electricity and Electrical Apparatus 

To determine the size of motor to use with a 
given machine, the best method is to make an actual 
test with the machine doing the maximum work it 
will be called upon to do. However, as it is not 
always practicable to make a test, resort must be 



Linotype (Type-Setting) Machine. 

had to tests recorded under average conditions of 
service. Practical experience, together with good 
common sense, will aid more than any hard and fast 
rule in predetermining the size motor to use in any 
special case, but the following list, giving size of 


262 


Motor Drive 


motors used in actual installations under average 
conditions, will be found useful as a basis of ref¬ 
erence. 



Fig. 276. — Sprague Round Type Motor, Direct- 
Connected to Electrotyping Dynamo. Special 
Example of Motor-Generator Set. 


Machine Tools. 

Machine H.P. of Motor 

72-in. driving-wheel lathe 5 

48-in. lathe 5 

36-in. lathe 4 

24-in. lathe 3 

16-in. tool-room lathe . 2 

60-in. x 60-in. x25-ft. Pond planer 15 

36-in. x 36-in. x 10-ft. Pond planer 5 

18-in. Cincinnati D.H. shaper 3 

18-in. Newton slotter 7\ 

14-in. Newton slotter 5 

5-ft. radial drill 5 

40-in. vertical drill 2 

4-spindle gang drill 7\ 

10-ft. Pond boring mill 20 

51-in. Baush boring mill 7\ 

No. 4 Newton duplex milling machine 10 

No. 1 Acme bolt cutter 7| 


263 


Electricity and Electrical Apparatus 


Machine 

H.P. of Motor 

No. 5 oscillating grinder 

25 

No. 2 oscillating grinder and twist drill grinder 5 

Car wheel press 

10 



Fig. 277. — Sprague Round Type Vertical Motor, Belted 
to Routing Machine. 



Fig. 278. — Grinding Wheels on Shaft of General 
Electric Induction Motor. 

Sawmill and Woodworking Machinery. 
Machine H.P. of Motor 

Automatic cut-off saw 10 

38-in. band resaw 8 


264 





Motor Drive 


Machine H.P. of Motor 

Automatic car gainer 15 

Buzz planer 7\ 

Planer and matcher 25 

Double surfacer 17J 

Four-sided timber planer 45 

Rip saw 15 

Scroll saw 2 

Tenoning machine 5 

Hollow chisel mortiser 4 

Universal saw bench 5 



Fig. 279. — Saw on Shaft of General Electric Type CQ 
Motor. 


Blacksmith Shop Machinery. 

Machine H.P. of Motor 


Bolt header 5 

Bolt shears 5 

Bradley hammer 5 

Forge blower 15 


265 



Y 


Electricity and Electrical Apparatus 

To bring home the relative cheapness of electric 
power, the following list has been compiled, show¬ 
ing what a nickel’s-worth of power at 10c. per 
K.W.-hour will do: 

Light a small room 75 minutes a night for a 
week. 

Operate a 12-inch fan for 7^ hours. 

Run a sewing machine for 15 hours. 

Keep a 6-pound electric flat-iron hot for 75 
minutes. 

Boil 10 quarts of water. 

Run a massage machine for nearly 20 hours. 

Run an electric pianola for 5 hours. 

Pump 2,500 gallons of water 50 feet high. 

Raise 50 tons 12 feet high with an electric crane. 

Raise a large passenger elevator eight stories 
three times. 

Heat an electric curling iron once a day for two 
months. 

Vulcanize four large patches on an automobile 
tire. 

Burn an ordinary carbon-filament lamp giving 
16-candle power for 9 hours. 

Burn an 80-watt tungsten lamp giving 64- 
candle power for 6J hours. 

Light a single glower Nernst lamp giving 85- 
candle power for 4^ hours. 

Burn an enclosed arc lamp giving 370 mean 
hemispherical candle power for 70 minutes. 

Light a 50-inch Cooper-Hewitt Mercury Vapor 
tube giving 500-candle power, for 80 
minutes. 


266 


CHAPTER XXVII 


OUTPUT 

Limitations — Efficiency — Tests — Fahrenheit and 
Centigrade Thermometers — Ratings — Guaran¬ 
tees. 

One would naturally suppose that the larger a 
machine, the greater its capacity. While this is 
true in a general way, yet it is not universally the 
case. For example, we might have two generators, 
apparently the same size and weight, one of which 
would easily handle a load of 500 K.W. and the 
other would be dangerously overloaded at 200 or 
250 K.W. 

Aside, then, from the size, there are several 
other conditions which limit the output of an electri¬ 
cal machine. As one illustration, suppose we could 
safely obtain 50 amperes at 110 volts or 5.5 K.W. 
from a given dynamo running at 1000 R.P.M. If 
we could increase the speed to 2000 R.P.M. the 
voltage would be doubled. We could still take 50 
amperes of current from the machine with equal 
safety, but in this case the power would be 50 X 220 
or 11.0 K.W. From this one illustration it is appa¬ 
rent why slow-speed machines are so much heavier 
and more expensive than those of the same rated 
output designed for high speeds. 

The factors which limit the output of a given 
machine are: 

Voltage 

Current 

Regulation 


267 


r 


Electricity and Electrical Apparatus 

Strength of those parts which transmit the 
power mechanically, and 

Available Power of the prime mover which is 
driving the machine, if a generator. 

These last two are self-evident, so we will proceed 
to study the conditions on which the other factors 
■— voltage, current, and regulation — depend. 

The greatest voltage which may be generated 
in a machine is limited by the strength of insulation 
of the windings, the speed, and strength of magnetic 
held. Because of that magnetic property of iron 
known as saturation (see Chapter XX) the current 
required in field windings to increase the strength 
of field above a certain point is disproportionately 
large. Also the amount of current that can be sent 
through these windings is limited, both by the re¬ 
sistance, and by the heat which large currents would 
produce in them. 

To get the greatest output from a given amount 
of material, the speed should be as high as possible. 
But it must not be so great as to destroy the com¬ 
mutator or armature by centrifugal force, or to 
cause any of the commutator bars to work loose and 
rise above the others; nor may it be so great as to 
overheat the bearings. If an alternating-current 
machine, the speed is fixed and determined by the 
frequency of the current it must supply. 

The amount of current w T hich may be taken 
from a machine is dependent upon the heating of 
the windings, and in direct-current machines, upon 
commutation. If a wire carries more than a certain 
amount of current for any considerable time, it will 
become so hot as to burn the insulation. Of course 
this amount depends on the kind of insulation and 
also on the location of wire. If out of doors, fanned 
by the cool breezes, obviously it can carry more 
current than if in doors, under a moulding or in a 
conduit. As armature windings are subjected to 
very forcible ventilation, due to the drafts of air 


268 


Output 


set up by rotation of the armature, the conductors 
may be allowed to carry more than wires of the same 
size under other conditions. 

Besides that generated in the winding, heat is 
produced in the iron core or body of the armature 
by eddy currents, which can not be entirely elim¬ 
inated even though the core is built up of thin 
laminations, and by hysteresis, a sort of magnetic 
friction in the iron caused by the rapid reversals of 
magnetism. The heat produced in the armature 
in these ways, must not be greater than can be 
dissipated before the temperature exceeds certain 
safe limits. 

The commutator also tends to Qverheat, when 
the load becomes excessive, because there is a cer¬ 
tain amount of resistance in the contact between 
commutator and brush. The energy consumed in 
this resistance appears as heat in the commutator. 
Again, heavy currents in the armature coils are more 
difficult to commutate, producing an excessive amount 
of sparking, which also heats the commutator, as 
well as burning it and making it rough and uneven. 

The regulation of a generator is the variation 
in voltage from no load to the rated full load, ex¬ 
pressed as a per cent, of full-load voltage. To 
understand why the regulation should limit the 
output of a generator, we must remember that for 
the usual commercial purposes, it is essential that 
the voltage should be very nearly constant, no 
matter what the load. You have often been on a 
car, when that and possibly several others at the 
same time were climbing a steep hill. The genera¬ 
tors and feeder system under those conditions were 
so heavily overloaded that the car lights were notice¬ 
ably dimmed. For railway motors such a reduction 
in the voltage is not important, but in a lighting 
system it would be intolerable. 

The regulation of a motor is its variation in 
speed from no load to full load, in per cent, of full- 


269 


Electricity and Electrical Apparatus 

load speed. The following discussion of the regu¬ 
lation of a motor will point out to us some of the 
factors which affect regulation in any dynamo, and 
then we will be prepared to note how the rated out¬ 
put of a machine is limited by the regulation. This 
discussion refers to an ordinary shunt motor on a 
constant potential circuit, with the field connected 
directly across the line. 

Under no load, the motor is, of course, running 
free, and the armature current and torque are small. 
The drop in voltage due to armature resistance will 
be small and the counter electromotive force high, 
almost equaling the line voltage. It must be re¬ 
membered that the sum of the resistance drop in 
the armature and the counter electromotive force 
is the line voltage, and that the torque is proportional 
to the armature current fnd field strength. 

As the load is put on, the torque is insufficient, 
and the speed drops, thereby decreasing the counter 
electromotive force and allowing the armature cur¬ 
rent to increase until the resistance drop in armature 
becomes the difference between the line voltage and 
the counter electromotive force. 

It will be readily seen from the following example 
that a small decrease of speed gives a large increase 
in armature current and torque. Therefore, the 
regulation of a shunt motor is, as a rule, good. 

For illustration, take a shunt motor with an 
armature resistance of .15 ohms connected to a 
115-volt line. When running free, the armature 
current is 5 amperes, and therefore the resistance 
drop in armature is .75 volts, and the counter electro¬ 
motive force is 114.25 volts. 

If the full-load current is 40 amperes, the 
resistance drop will be 6 volts and the counter 
electromotive force will be 109 volts. 

With a no-load speed of 1500 revolutions per 
minute, it is apparent that the speed under full load 
is approximately: 


270 


Output 


1500 X Yia ok = 1430 revolutions per minute. 

The example is merely to make clearer this drop 
in speed as the load comes on, and if applied to a 
practical case, other quantities, such as armature 
reaction, and the drop due to contact resistance of 
brushes and commutator, would still further affect 
the speed. At heavy loads, armature reaction plays 
a very important part. 

In the case of a shunt generator, these same 
causes produce a reduction in the voltage. As the 
voltage decreases, the field current is reduced. This 
causes a still further reduction in the voltage. If 
the load on a shunt generator were increased to 
extremely large values, and other parts did not first 
give way or burn out, the effect of this would be that 
finally the machine would cease to generate. Char¬ 
acteristic curves of a shunt generator droop very 
rapidly beyond 25% or 50% overload. 

A well-designed machine should reach the 
limits of satisfactory operation in each of these 
particulars at about the same load. That is, when 
the temperature rise in armature reaches the value 
specified in the guarantee, that in the commutator 
should be at or near its specified limit; loss of speed 
or voltage should be no greater than permissible in 
the service to which machine is to be applied; effi¬ 
ciency^ should be at or near its maximum; speed 
should be as high as consistent with other require¬ 
ments; and field magnets should be worked at or 
near their points of saturation. Such conditions 
insure the greatest output for the amount of ma¬ 
terial used. 

Electrical machines are subjected to tests for 
several purposes. The usual tests applied are those 
for determining the efficiency and other data for 
characteristic curves, for proving that there are no 
serious defects in the insulation, and for determin- 


271 



Electricity and Electrical Apparatus 

ing the rise in temperature of various parts of the 
machine under varying conditions of load. 

The efficiency of any machine is the ratio of 
the output to the energy supplied. In other words, 
the greater the amount of losses in a machine, the 
less will be its efficiency. These losses can be 
divided under two headings: 

1. Stray power, consisting of losses due to 
bearing friction, friction of brushes, if any, on com¬ 
mutator or slip rings, windage, or air resistance to 
rotation due to the fanning action of rotating ele¬ 
ments, and lastly, core losses, from eddy currents 
and hysteresis in the iron. 

2. Electrical losses, or RP loss in armature 
and field windings, brushes, etc. 

Efficiency tests may be made in several ways. 
We may run the machine as a motor and measure 
the amount of electrical energy supplied to it, at 
the same time determining its power output by 
means of a “prony brake,” or we may use the so- 
called motor-generator method, or thirdly the stray- 
power method. 

Insulation tests are applied for the purpose of 
discovering any weak points in the insulation before 
the apparatus is sent out of the shops. While the 
ohmic resistance should be large, it is a less im¬ 
portant point than the dielectric strength or resist¬ 
ance to rupture at high voltage. 

High potential should be applied from each 
winding to all other windings, and then from each 
winding to the frame. In all ordinary cases, small 
motors for operation on circuits of 800 volts or .less, 
and in sizes up to and including ten horsepower, a 
test voltage of 1500 is sufficient, while on fifteen 
horsepower and larger, 2000 volts is used. 

The breaking down of the insulation of a motor 
may be attended with results more or less surpris- 
ing, of which the following incident will serve as an 
example. Mounted on an insulated stand in a 


272 


Output 


dentist’s office, was a small two-phase induction 
motor, used in connection with a dental drill. There 
were four incoming lines to the motor, two for each 
phase, and where they entered the building, one of 
the lines to phase A was grounded. Now it so 
happened that at the motor the other side of the 
same phase had given way, allowing the winding 
at this point to make contact with the motor frame 
and thus through the bearings and flexible shaft 
to the drill. If, while the drill was in operation, the 
patient touched the metal cuspidor, preparatory to 
expectorating, the sensations received were hardly 
pleasing, and caused doubt as to the real meaning 
of the signs in front, advertising painless treatment. 

Needless to say there were several excited 
patients and a much mystified dentist before the 
real cause of the trouble was revealed by a volt¬ 
meter being connected from the point of the drill to 
the metal cuspidor, which was grounded by the 
piping. 

In the case of a transformer, two tests are made. 
In the first, a voltage is applied from secondary to 
all parts (frame and other winding), and then a 
voltage is applied from the primary to all parts. 
How much this voltage should be, depends on the 
circumstances and the rated voltage of the trans¬ 
former. 

Recommendations from the Standardizations 
Rules of the American Institute of Electrical Engi¬ 
neers are that this voltage should be 10,000 volts for 
transformers with primary pressures of from 550 to 
5,000 volts, and with their secondaries connected 
directly to consumption circuits. 

Second test consists in applying to one of the 
windings, a voltage two or three times normal. The 
other winding is left open-circuited. This of course 
makes two or three times normal voltage between 
all the different layers and turns of each winding. 

Electrical machines are usually sold under a 


273 


Electricity and Electrical Apparatus 


guarantee that the various parts will not rise more 
than a specified number of degrees above the tem¬ 
perature of the air, after running under a given load 
for a certain time. To make sure that these re¬ 
quirements are fulfilled, the machine is subjected 
to various loads for the time called for in the guaran¬ 
tee, and at the end of that time, the temperatures 
of various parts are measured and recorded. The 
temperatures of the coils are best determined by 
measuring their resistances at the start of the run 
and at its finish. As copper increases in resistance 
T % of 1% for each degree Centigrade rise in tem¬ 
perature, it is seen that the average temperature of 
interior of the coil may be determined by this method, 
which is much better than to measure the tempera¬ 
ture at the outside of the coil with a thermometer. 
However, this method cannot be applied to such 
parts as the commutator or bearings, and for these 
parts thermometers must be used. After the ma¬ 
chine has stopped, the thermometer bulb is laid 
against the commutator, covered with a small piece 
of clean, dry felt, and held until it reaches a constant 
temperature. Sometimes, especially in such places 
as on the bearings or other stationary parts of the 
machine, the thermometer bulb is attached to the 
machine while running, by a wad of putty. 

In this connection we note that there are two 
kinds of thermometers in use in this country: the 
Fahrenheit, which is in common use, and the Centi¬ 
grade, which is more largely used in engineering 
and scientific work. 5° on the Centigrade thermom¬ 
eter is equal to 9° on the Fahrenheit. The Centi¬ 
grade thermometer has its zero point at the tem¬ 
perature of melting ice, while the temperature of 
steam or boiling water is 100° on this scale. These 
points on the Fahrenheit thermometer are 32° and 
212° respectively. By keeping these figures in mind 
one can easily reduce temperatures given on one 
scale to those indicated by the other. 


274 


Output 


The following is an extract from the report of 
Standardizations Committee of American Institute 
of Electrical Engineers, on the allowable tempera¬ 
ture rise and overload capacities of electrical appa¬ 
ratus, as affecting the ratings and guarantees. 



Fig. 280. — Fahrenheit and Centigrade Thermometers 
Compared. 


It is recommended that the- following maximum values 
of temperature elevation should not be exceeded after con¬ 
tinuous operation at full load: 

Commutating machines, rectifying machines and syn¬ 
chronous machines. 

Field and armature, by resistance, 50° C. 

Commutator and collector rings and brushes, by 
thermometer, 55° C. 

Bearings and other parts of machine, by ther¬ 
mometer, 40° C. 

Rotary induction apparatus. 

Electric circuits, 50° C., by resistance. 

Bearings and other parts of machine, 40° C., by ther¬ 
mometer. 


275 

























































Electricity and Electrical Apparatus 


In squirrel-cage or short-circuited armatures, 55° C., by 
thermometer, may be allowed. 

Transformers for continuous service — electric circuits 
by resistance, 50° 0., other parts by thermometer, 40° C., 
under conditions of normal ventilation. 

Reactors, induction and magneto-regulators — electric 
circuits by resistance, 50° C., other parts by thermometer, 
40° C. 

Where a thermometer, applied to a coil or winding, in¬ 
dicates a higher temperature elevation than that shown by 
resistance measurement, the thermometer indication should 
be accepted. In using the thermometer, care should be taken 
so to protect its bulb as to prevent radiation from it, and, at 
the same time, not to interfere seriously with the normal 
radiation from the part to which it is applied. 

In the case of apparatus intended for intermittent service, 
except railway motors, the temperature elevation which is 
attained at the end of the period corresponding to the term 
of full load, should not exceed 50° C., by resistance in electric 
circuits. In the case of transformers intended for inter¬ 
mittent service, or not operating continuously at full load, 
but continuously in circuit, as in the ordinary case of light¬ 
ing transformers, the temperature elevation above the sur¬ 
rounding air temperature should not exceed 50° C. by re¬ 
sistance in electric circuits and 40° C. by thermometer in other 
parts, after the period corresponding to the term of full load. 
In this instance, the test load should not be applied until the 
transformer has been in circuit for a sufficient time to attain 
the temperature elevation due to core loss. With trans¬ 
formers for commercial lighting, the duration of the full-load 
test may be taken as three hours, unless otherwise specified. 
In the case of railway, crane, and elevator motors, the con¬ 
ditions of service are necessarily so varied that no specific 
period corresponding to the full-term load can be stated. 

The commercial rating of a railway motor should be the 
H.P. output giving 75° C. rise of temperature above a room 
temperature of 25° C., after one hour’s continuous run at 
500 volts terminal pressure, on a stand, with the motor covers 
removed. 

Overload Capacities. 

All guarantees on heating, regulation, sparking, etc., 
should apply to the rated load, except where expressly speci¬ 
fied otherwise, and in alternating-current apparatus to power 
factor = 1. 

All apparatus should be able to carry the specified over¬ 
load without self-destruction by heating, sparking, mechani¬ 
cal weakness, etc., and with an increase in temperature eleva¬ 
tion not exceeding 15° C. above those specified for full loads, 


276 


Output 


the overload being applied after the apparatus has acquired 
the temperature corresponding to full-load continuous opera¬ 
tion. 

Overload guarantees should refer to normal conditions 
of operation regarding speed, frequency, voltage, etc., and to 
power factor = unity. 

The following overload capacities are recommended: 

1st. In direct-current generators and alternating-current 
generators, 25 per cent, for two hours. 

2d. In direct-current motors, induction motors and 
synchronous motors, not including railway motors and other 
apparatus intended for intermittent service, 25 per cent, for 
two hours, and 50 per cent, for one minute, for momentary 
overload capacity. 

3d. Synchronous converters. 50 per cent, for one-half 
hour. 

4th. Transformers. 25 per cent, for two hours. Ex¬ 
cept in transformers connected to apparatus for which a 
different overload is guaranteed, in which case the same 
guarantees shall apply for the transformers as for the appara¬ 
tus connected thereto. 

5th. Exciters of alternators and other synchronous 
machines, 10 per cent, more overload than is' required for the 
excitation of the synchronous machine at its guaranteed over¬ 
load, and for the same period of time. 

7th. All exciters of alternating-current single-phase or 
polyphase generators, should be able to give, at constant speed, 
sufficient voltage to excite the alternator, at the rated speed, 
to the full-load terminal voltage, at the rated output in kilo¬ 
volt amperes and with 50 per cent, power factor. 


277 


CHAPTER XXVIII 

WIRES 

Copper, Aluminum, etc. — Weight — Resistance — 

Current-Carrying Capacity ■— The Wire Table. 

In buying and ordering wires for work of any 
importance, such as for transmission lines, it is 
necessary to decide what size, and kind of wire would 
be satisfactory. This brings up the question of how 
wires are measured. The different sizes of wire 
could be specified by naming their diameters, as, 
for instance, a wire three-tenths (.3) or twenty-three 
one-hundredths (.23) or fifty-five one-thousandths 
(.055) of an inch in diameter, these measurements 
being taken with an ordinary micrometer. 

The unit most commonly used, however, in 
measuring wires, is the mil, this being a short name 
for a thousandth of an inch. Thus, if we say a 
certain wire is 18 mils in diameter, we mean that 
it would measure (.018") eighteen thousandths of 
an inch. 

In measuring the cross-sectional area of a round 
wire, we use a unit called the circular mil, this being 
the area of a circle whose diameter is one mil. If 
we have a square whose sides measure 2 inches, we 
find its area by multiplying 2X2. That is, the 
square contains 4 square inches. If its sides were 
2 mils, the area would be 2 X 2 = 4 square mils, or 
.000004 square inch. In a similar way, a circle 
whose diameter is 2 mils, would have an area of 
2X2 = 4 circular mils, written 4 CM*. 

* The area in square mils is equal to that in circular 
mils multiplied by ^ or .7854, while the area in square 


278 


Wires 


For convenience, the size of wire is commonly 
specified in this country by a certain gauge number 
instead of by the diameter in inches or in mils. The 
larger sizes of cable, however, are listed as having a 
cross-section of so many circular mils. In the 
United States the Brown & Sharpe (B. & S.) Gauge 
is in almost universal use. Thus a No. 18 wire is 
understood to mean No. 18 B. & S. Gauge, which 
is 40 mils in diameter. In England, the gauge most 
universally used is the “New British Standard Gauge. ’ ’ 
For the convenience of those using wires very 
much and constantly in need of information regard¬ 
ing them, wire tables have been prepared. These 
are merely a compilation of data and information 
about wires of different materials giving the dia¬ 
meter in mils, cross-sectional area in circular mils, 
weight and resistance per 1,000 feet of the various 
sizes. Most of the data is calculated from the formu¬ 
las which may be found later in this chapter. 

In the United States and Great Britain,, the 
length of a wire is measured by the inch, foot or mile. 
In France and other countries where the metric 
system of measuring is employed, the length of a 
wire is measured by centimeters, meters and kilo¬ 
meters. The metric system of measurements is 
much simpler and has other advantages over the 
system we use in this country. It has not been 
extensively adopted in every-day work in the United 
States, except in our system of money, although 
widely used in scientific work. Tables are given 
in the Appendix showing values of the units used in 
the metric system in terms of those used in the 
English or American system. 

inches equals that in circular mils times .0000007854. Pos¬ 
sibly the following table will be useful: 

1 square inch = 1,000,000 square mils. 

1 square inch = 1,275,000 circular mils. 

1 circular mil = .7854 square mil. 

1 circular mil = .0000007854 square inch. 


279 


Electricity and Electrical Apparatus 


A mil-foot is a length of wire one foot long and 
one mil or one-thousandth of an inch in diameter. 
One mil-foot of copper wire weighs approximately 
.00000303 pound, and 1000 ft. of wire, one mil in 
diameter, would weigh .00303 pound. Aluminum 



c - 


Fig. 281. — “Parac” Solid Rubber-Covered Wire. 

wire weighs about .00000105 pound per mil-foot, 
or .00105 pound per thousand mil-feet. w 

Knowing the weight of a wire 1000 feet long 
and 1 mil in diameter or 1 circular mil in area, we 
can calculate the weight of any wire by multiplying 
this weight, .00303 pound, by the area in circular mils. 
The result is the weight per 1000 feet. For copper 
wire, the formula is: 

W = .00303 D 2 

Where W = weight per 1000 feet 

D = diameter of wire in mils 

or W = .00303 A 

where A = area of wire in circular mils. 



Fig. 282. — “Parac” Stranded Rubber-Covered 
Conductor. 

From this formula the weight of any amount of 
copper wire can be found, if the length in feet and 


280 





Wires 


diameter in mils is known. For an example, No. 18 
copper wire is 40 mils in diameter. 1000 feet would 
weigh W = .00303 X 40 X 40 = 4.85 pounds; 200 
feet would weigh 4.85 X .200 = .97 pound. 

The resistance of one mil-foot of copper wire 
of ordinary commercial purity and at a temperature 



Fig. 283. — “ Parac "Duplex Rubber-Covered Wires, for 
Use in Conduit, etc. 

of about 25° C. or 75° F., is approximately 10.5 
ohms, and this value can be used in preliminary 
calculations and where extreme accuracy is not 
essential. 

Using this value, the resistance of a copper wire is 


R = 


10.5 L 
D 2 


Where R = resistance of the wire 
L = its length in feet 
D = its diameter in mils 


or 

where 



L 


A = its area in circular mils. 



Fig. 284. — “O.K." Solid Weatherproof. 

To apply this formula, suppose it is desired to 
find the resistance of a No. 16 wire 1000 feet long. 


281 








Electricity and Electrical Apparatus 


The diameter is 51 mils, or the area is 51 2 = 2601 
circular mils. Substituting these figures in the 
formula 


R = 


10.5 X 1000 
2601 


= 4.04 ohms. 


If greater accuracy is necessary, we must take 



Fig. 285. — “O.K.” Slow-Burning Weatherproof, Black 
Outside. 

into account the temperature of the wire, and use 
instead of 10.5, 9.57 times 1 plus the temperature 
in degrees Centigrade multiplied by .004 (9.57 X 

[1 + .004^]). The reason for this is that metallic 
wires, such as copper or iron, expand when heated. 
The molecules or little particles of which the wire 
is composed push each other further apart as the 
temperature rises, causing the wire to expand and 
at the same time making it more difficult for the 
current to pass from molecule to molecule — in other 



Fig. 286. — “O.K.” Slow-Burning, White Outside. 

words, increasing the resistance. A thousand feet 
of No. 16 copper wire has a resistance of 4.04 ohms 
at 75° F. as compared with only 3.91 at 60° F. 

Wires for electrical use may be made, according 
to the purpose for which they are intended, of copper, 
iron, steel, german silver, or any one of numerous 


282 







Wires 


alloys of two or more metals, many of these alloys 
having special trade names such as Nichrome, Man- 
ganin, Climax, Superior, “la la,” Nickeline, etc. 

Copper is used most extensively for the trans¬ 
mission of current, and in the current-carrying parts 
of electrical apparatus where low resistance is de¬ 
sired. It has the lowest resistance of any metal 
except silver, and nearly as low as that. It is 
reasonably strong and at the same time pliable, easy 
to twist and join together. 

Being lighter than copper for the same con¬ 
ductivity, aluminum is sometimes used, although it 


Fig. 288. — Paper-Insul¬ 
ated, Lead-Covered Cable, 
with Jute and Asphalt 
Jacket. G. E. Co. 

is very brittle and hard to handle, and although 
lighter it is larger in size than copper and more ex¬ 
pensive to cover with insulation. Where only a 
small current is to be transmitted, as in telegraph 
work, iron wires are sometimes used. Their ad¬ 
vantage is in strength and cost. The conductivity 
of iron is about J that of copper, hence a copper wire 
of a given size can carry approximately six times as 
much current as an iron of the same size. 

There are on the market several bimetallic 
wires, consisting of an iron wire in the center 



Fig. 287. — Paper-Insul¬ 
ated and Lead-Covered 
Cable. G. E. Co. 



283 



Electricity and Electrical Apparatus 

surrounded by a copper cover. This gives the 
strength of the iron combined to a degree with the 
conducting properties of copper. Its higher initial 
expense offsets these advantages for most uses. 

For use in rheostats or in other places where 
high resistance is desired, german silver or some 
other high-resistance alloy is used. For standards 
of resistance, which must be constant regardless of 
temperature conditions, a wire of manganin is often 
used, as this alloy between certain limits has almost 



Fig. 289. —Paper-Insulated, 
Lead-Covered and Band- 
Steel Armored Cable, with 
Jute and Asphalt Jacket. 
General Electric Co. 



Fig. 290. — Rubber-Insul¬ 
ated and Lead-Covered 
Cable. Four Conduct¬ 
ors. General Electric 
Co. 


no variation in resistance with changes in tempera¬ 
ture. 

In cases where the current is large, wire of 
sufficient cross-section must be used to prevent 
overheating, and to reduce the voltage as little as 
possible. In handling wires, stringing them on 
poles, etc., it is hard to handle a hard, stiff wire — 
being awkward to make joints and bends — conse¬ 
quently No. 0000 B. & S. gauge is the largest single 
or solid wire ordinarily used. After this point cables 
are used, consisting of a number of smaller wires 
stranded together. 


284 


Wires 


To insulate wires from each other and prevent 
short-circuits or grounds and endangering lives, 
wires are covered with insulation. There are several 
kinds in use, the three most common being Rubber 
Covered, Weatherproof, and Slow-Burning Weather¬ 
proof ; and for underground and submarine work, 
lead-covered cables are often used. 

In rubber-covered wires, usually called R.C., 
the copper conductor is first tinned, and then cov¬ 
ered with rubber, over which is placed a finishing 
layer of cotton braid, saturated with a weatherproof 



Fig. 291. — Rubber-Insul¬ 
ated, Lead-Covered and 
Wire-Armored Cable. 
General Electric Co. 



Fig. 292.—Paper-Insulated 
and Lead-Covered Cable, 
Three Conductors. 
General Electric Co. 


compound. This protects the rubber insulation. 
Rubber-covered wires are used largely in indoor work. 

In the weatherproof insulation, the bare wire is 
covered with three braids, which are then thoroughly 
impregnated with a black weatherproof compound. 
The outside is polished or rubbed smooth so as to 
make it shed water or snow more easily. This is 
known as triple-braided. Wires with this insulation 
are particularly applicable for exterior work. 

Slow-burning weatherproof is suitable for in¬ 
door use in certain cases, and is required where a 


285 


Electricity and Electrical Apparatus 


large quantity of small wiring is bunched, as behind 
switchboards, or in wire shafts. Here rubber-cov¬ 
ered wire would cause a large fire risk. It also has 
three braids, all of which are thoroughly soaked in 
a flame-proof compound and then polished. 


CURRENT-CARRYING CAPACITY 

Solid Wires 

Cables 

Size 

Amperes 

Amperes 

Size 

Amperes 

Rubber 

Amperes 


Rubber 

Weather- 


Weather¬ 

Gauge No. 

Covered 

proof 

Area 

Covered 

proof 

B. & S. 

Insulation 

Insulation 

Cir. Mils. 

Insulation 

Insulation 

18.... 

3 

5 

300,000 

270 

400 

17.... 

4 

6 

400,000 

330 

500 

• 16.... 

6 

8 

500,000 

390 

590 

15.... 

8 

10 

600,000 

450 

680 

14.... 

12 

16 

700,000 

500 

760 

13.... 

14 

19 

800,000 

550 

840 

12.... 

17 

23 

900,000 

600 

920 

11. ... 

21 

27 

1,000,000 

650 

1000 

10.... 

24 

32 

1,100,000 

1,200,000 

690 

1080 

9.... 

29 

39 

730 

1150 

8. ... 

33 

46 

1,300,000 

770 

1220 

7. . . . 

39 

56 

1,400,000 

810 

1290 

6.... 

45 

65 

1,500,000 

850 

1360 

5. . . . 

54 

77 

1,600,000 

890 

1430 

4. . . . 

65 

92 

1,700,000 

1,800,000 

930 

1490 

3.... 

76 

110 

970 

1550 

2. . . . 

90 

131 

1,900,000 

1010 

1610 




2,000,000 

1050 

1670 

1. .. . 

107 

156 



0 .... 

127 

185 




00.... 

150 

220 




000.... 

177 

262 




0000.... 

210 

312 





Cables for submarine or underground work are 
lead-covered. In some cases the lead covering is 
protected by a layer of iron wires. 

The more current a wire carries, the hotter it 
becomes. Now if the temperature of a conductor 
rises beyond a certain degree it will be hot‘enough 


286 


















Wires 


to cause a deterioration of the insulation — burning 
or charring it. Therefore, there is a certain current 
limit which should not be exceeded. This is called 
the current-carrying capacity of a wire, and in the 
accompanying wire tables the values of current 
given are those which the conductors will carry 
safely. It will be noticed that the current values 
are less for rubber-covered wire than for weather¬ 
proof, as rubber is more quickly affected by heat 
and the wires must not be allowed as high a tem¬ 
perature rise as with weatherproof. 

It must be remembered that the heating effect 
varies as the square of current. That is, if the cur¬ 
rent is doubled the heating effect is four times as 
great. 

The following table is convenient to find differ¬ 
ent sizes of wire which can be used in case the de¬ 
sired size is not at hand. For instance, if you wished 
to use a No. 0000 and had none at hand, 8 No. 6 
wires would serve the same purpose. 


Wires of Equivalent Cross Section. 


0000 

2- 0 

4- 3 

8- 6 

16- 9 

32-12 

64-15 

000 

2- 1 

4- 4 

8- 7 

16-10 

32-13 

64-16 

00 

2- 2 

4— 5 

8- 8 

16-11 

32-14 

64-17 

0 

2- 3 

4- 6 

8- 9 

16-12 

32-15 

64-18 

1 

2- 4 

4- 7 

8—10 

16-13 

32-16 


2 

2- 5 

4- 8 

8-11 

16-14 

32-17 


3 

2- 6 

4- 9 

8-12 

16-15 

32-18 


4 

2- 7 

4-10 

8-13 

16-16 



5 

2- 8 

4-11 

8-14 

16-17 



6 

2- 9 

4-12 

8-15 

16-18 



7 

2-10 

4-13 

8-16 




8 

2-11 

4-14 

8-17 




9 

2-12 

4-15 

8-18 




10 

2-13 

4-16 





11 

2-14 

4-17 





12 

2-15 

4-18 





13 

2-16 

4-19 






14 2-17 

15 2-18 

16 2-19 


287 


Electricity and Electrical Apparatus 




Wire 

Table 



Gauge No. 
B. & S. 

Diameter 

Mils. 

Area 

Circular 

Mils. 

Ohms per 

1000 Ft. 

Lbs. per 

1000 Ft. 

Bare 

Lbs. per 

1000 Ft. T. 

B. Weather¬ 

proof Insu¬ 
lation 

18. . . 

40 

1,600 

6.3880 

4.92 

16 

17. . . 

45 

2,025 

5.0660 

6.20 


16. . . 

51 

2,601 

4.0176 

7.82 

20 

15. . . 

57 

3,249 

3,1860 

9.86 


14. . . 

64 

4,096 

2.5266 

12.44 

26 

13. . . 

72 

5,184 

2.0037 

15.68 


12. . . 

81 

6,561 

1.5890 

19.77 

35 

11. . . 

91 

8,281 

1.2602 

24.93 


10. .. 

102 

10,404 

.99948 

31.44 

55 

9. . . 

114 

12,996 

.79242 

39.65 


8. . . 

128 

16,384 

.62849 

49.99 

78 

7. . . 

144 

20,736 

.49845 

63.03 


6. . . 

162 

26,244 

.39528 

79.49 

112 

5. . . 

182 

33,124 

.31346 

100.23 

145 

4. . . 

204 

41,616 

.24858 

126.40 

164 

3. . . 

229 

52,441 

.19714 

159.38 

210 

2. 

258 

66,564 

.15633 

200.98 

268 

1. . . 

289 

83,521 

.12398 

253.43 

306 

0 ... 

325 

105,625 

.09827 

319.74 

400 

00. . . 

365 

133,225 

.07797 

402.97 

490 

000. . . 

410 

168,100 

.06134 

508.12 

630 

0000... 

460 

211,600 

.04904 

640.73 

775 

Cables . 

630 

300,000 

.03355 

932 


it 

727.3 

400,000 

.02516 

1242 


“ 

814.5 

500,000 

.02013 

1553 


a 

891.9 

600,000 

.01666 

1863 

% § 

it 

963.9 

700,000 

.01438 

2174' 

> o 
m IS 

a 

1030.5 

800,000 

.01258 

2474 

.2 

a 

1092.6 

900,000 

.01118 

2795 

> § 

it 

1152 

1,000,000 

.01006 

3106 

^ c 

GO 

it 

1208.7 | 

1,100,000 

.00915 

3416 

i>«4H 

o o 

it 

1262.8 

1,200,000 

.00838 

3727 

cn 

it 

1314.5 J 

1,300,000 

.00769 

4038 

Ch 

it 

1364 

1,400,000 

.00715 

4348 


a 

1413.5 

1,500,000 

.00667 

4658 

fa 

a 

1458.6 

1,600,000 

.00625 

4968 

be & 

a 

1503.7 

1,700,000 

.00588 

5278 


a 

1547.7 

1,800,000 

.00556 

5588 

of 

a 

1571.9 

1,900,000 

.00527 

5898 

EH 

i t 

1630.2 ! 

2,000,000 

.00500 

6208 



288 























Wires 


Properties of Metals and Alloys 

Metal 

Resist¬ 
ance per 
Mil Foot 

Weight 

per 

Mil Foot 

Weight 

per 

Cu. Inch 

Melting 

Point 

Degrees 

Fahrenheit 

Copper 

10.5 

.00000303 

.3195 

2000 

Brass 

42 

.00000285 

.300 

1700 

German Silver 

127 

.00000290 

.307 

.... 

Aluminum 

19 

.00000091 

.096 

1200 

Iron (wrought) 

64 

.00000264 

.278 

2800 

Platinum 

56 

.00000738 

.778 

3227 

Mercury 

568 

.00000465 

.490 

-39 

Nickel 

77 

.00000302 

.318 

.... 

Zinc 

37 

.00000240 

.253 

750 


Brass is an alloy of two parts copper and one 
part zinc. 

German silver is composed of copper, 4 parts; 
nickel, 2 parts; and zinc, one part — by weight. 

These proportions are varied by different manu¬ 
facturers and for different purposes. 


289 












CHAPTER XXIX 


PROTECTIVE DEVICES 

Fuses — Circuit Breakers — Oil Switches 

The tables in the foregoing chapter give values 
of current which should not be exceeded for per¬ 
fectly safe operation. The question now arises: 
How shall we keep more than that amount from 
flowing? If more lamps are connected, more cur¬ 
rent will flow; and the only way to prevent this is 
to open the circuit altogether. 

Also electric wires stand a great many chances 
of being interfered with, or in some way of coming 
into accidental contact with each other, or with the 
ground, where the wire is bare or the insulation weak. 
The result is a “short circuit,” which causes an ex¬ 
cessive current to flow from the generator through 
the wires. It would not be practicable to have a 
man stand at the switch-board all the time and open 
the switch whenever such a thing occurs, so we have 
to provide some automatic device to prevent these 
excessive rushes of current. 

There are two general types of device for this 
purpose, one depending on the heating effect of the 
current, the other on its electromagnetic effect. A 
device of the first type is called a fuse, one of the 
second is called a circuit breaker. Either of these 
will open the circuit in case the current exceeds a 
certain predetermined value. 

A fuse is a wire or strip of soft metal, usually 
composed of lead, with small quantities of antimony 
and bismuth, which is inserted in the circuit so that 
the whole current which flows through the wire it 


290 


Protective Devices 


is desired to protect, flows also through the fuse. 
Under normal loads the current flows without in¬ 
terruption. But if the current rises above normal, 
sufficient heat is developed to melt the fuse. The 
fuse resistance is always greater, the cross-sectional 
area less, and the melting temperature much lower 
than that of copper. Consequently long before 
the wire reaches a temperature at all dangerous to 
the insulation the fuse melts and opens the circuit. 

Originally fuses were mounted in the form of a 
wire or strip between two copper or brass terminal 
blocks to which the wires were attached. The 
terminal blocks, together with the slate, marble or 
porcelain base on which they are mounted, is termed 
a “cut-out,” and a fuse so used is called an “open 



Fig. 293. 


link.” As a rule, fuse cut-outs are installed indoors, 
and on account of the fire risk from arcing and from 
melted metal and inflammable gases produced when 
a fuse “blows,” they must be designed so as to pre¬ 
vent danger from these sources. There are on the 
market many kinds of cut-outs, designed to take an 
open-link fuse, and having a cover or other means 
of protection to prevent the melted metal from fly¬ 
ing. Nearly all of these cut-outs will break and are 
a source of danger when the fuse blows under a very 
heavy overload or a short-circuit. 

To eliminate the dangers incident to open link, 
the enclosed or cartridge fuse shown in Fig. 294 is 
widely used and is required by the board of fire 
underwriters whenever the cut-out is not mounted 


291 









Electricity and Electrical Apparatus 

in a fire-proof cabinet. In sizes up to 60 amperes, 
the copper ferrules shown on each end of this fuse 
fit into spring-contact clips. From 61 to 600 am¬ 
peres, the fuses are provided with a copper blade at 
each end. The blades fit into clips similar to those 
of a knife switch. A blown fuse is thus easily with- 



Fig. 294. — Enclosed Fuse, Ferrule Contact. 
General Electric Co. 

drawn and a good one quickly substituted. The 
fuse itself is connected between the two copper 
blades and enclosed in the fibre tube. The space 
between fuse and tube is packed with a fire-proof 
powder, usually asbestos or silicate of magnesia. 
This powder must be of a character that will absorb 
the gases and prevent an explosion. When this 
fuse blows, there is no noise or arcing as in the case 
of the open link, which often blew with a loud re- 



Fig. 295. — Enclosed Fuse, Knife-Blade Contact. 
General Electric Co. 

port and with flashes which burned and blackened 
the switchboard on which they were mounted. 

The form most commonly used in residential 
work is known as a fuse plug. The fuse is mounted 
in a brass and porcelain plug designed to screw into 
an ordinary Edison socket like a lamp, and is pro¬ 
tected with a mica cover. Used as they are for 


292 


Protective Devices 


small capacities, they are not packed with silicate 
of magnesia. They are easily replaced, and being 
comparatively inexpensive, when blown the whole 
plug is thrown away. Plug fuses are ordinarily 
made in capacities ranging from 3 to 30 amperes. 
Cartridge fuses are made in sizes up to 600 amperes. 



Fig. 296. Fig. 297. 

Enclosed Fuses and Cutouts. 

There is a special type of fuse for use in 
high potential circuits. If the enclosed fuses 
previously described were employed on circuits of 
over 600 volts, there would be danger of flashing 
over from one copper terminal blade to the other, 
and maintaining the current through this arc even 



Fig. 298. Fig. 299. 

Plug Fuse and Cutout. 

after the fuse melted. This fuse, called the ex¬ 
pulsion type, consists of a cannon-shaped tube, open 
at the small end, in which is stretched the fusible 
wire. The surrounding space in the tube is empty, 
and the gases formed by the melting fuse are ignited, 
and the explosion blows out the arc and expels the 
gases in the direction in which the tube points. 


293 




Electricity and Electrical Apparatus 


Enclosed fuses are made so that they will carry 
indefinitely a current 10% greater than that at 
which they are rated when the surrounding tempera¬ 
ture is 75° F. or less, and with 25% overload they 
will open the circuit. With 50% overload, starting 
cold, they must blow within a certain specified time, 
depending on the capacity — two minutes for a 100- 
ampere fuse. Thus a 100-ampere fuse is one which 
will carry 110 amperes indefinitely, will blow after 
carrying 125 amperes for some time, and will open 
the circuit within 120 seconds if 150 amperes flow 
through it. If 300 amperes, causing four times as 
much heating, should flow, the fuse would blow 
within 30 seconds, or one-fourth of the time. 

An open link fuse is necessarily more or less 
variable, and its rating is less dependable. If in¬ 
stalled in a cool place or in a draft, the heat will be 
dissipated more quickly, hence it will carry larger 
currents for a longer time than if in a warm, pro¬ 
tected place. Fuse wire should not be clamped 
directly under the fuse screws, as such fuses often 
blow from heating due to poor contact with the 
clamping screw rather than from overload, also the 
cross-section of the wire is liable to be reduced. The 
fuse wire should be soldered to copper terminal clips, 
the clips then being clamped under the screws. 

Fuses should be large enough to carry from 25% 
to 50% more than the maximum current when all 
lights are on, but in no case larger than the capacity 
of the wire. This gives a margin so that fuses will 
not blow under normal current, and still protect the 
system from abnormal conditions. All fuses and 
cut-outs should be plainly marked with the voltage 
and current for which they are designed. 

If overloads or short-circuits are frequent, fuse 
renewals would become an item of considerable ex¬ 
pense, as well as causing delay in getting current on 
the line again. Where this is likely to be the case, 
circuit-breakers are used. 


294 


Protective Devices 


A circuit-breaker is a device which uses the 
electro-magnetic energy of an overload rather than 
its heating effect to open the circuit, which it does 
without itself being destroyed or injured. It may 
best be described as a 


switch, the tendency of 
which at all times is to 
spring open, but is held in 
contact by a restraining 
latch. (See Fig. 300.) A 
coil is arranged to be en¬ 
ergized by the line cur¬ 
rent, with a plunger which 
is drawn up into the coil 
when the current is large 
enough to lift it from its 
seat. As the plunger rises, 
it comes into a stronger 
part of the magnetic field, 
and by the time it strikes 
the latch it has acquired consid¬ 
erable speed — enough to trip 
the latch and allow breaker to 
open. The knurled screw is to 
adjust the plunger for various 
currents. Thus if the breaker 
will open on 100 amperes and 
we wish it to open on 85, we 
raise the plunger into a stronger 
part of the magnetic field. 

Fig. 301 shows a single pole 
I-T-E circuit breaker. The ad¬ 
justing feature is plainly seen, 
also the solenoid, which consists 
of heavy bar copper because 
of the large current it must carry. The cir¬ 
cuit is opened by the contacts at the top, which 
are of the knife-blade type. Carbon blocks are 
arranged to hold the connection until after the 



Fig. 300. 


295 

















Electricity and Electrical Apparatus 

copper blades have left the contacts, thus taking the 
arc when circuit is finally opened, and protecting 
the copper parts. 

Figure 302 shows a circuit-breaker of the lamin¬ 
ated type. The leaves or laminations of copper are 
pressed against flat copper terminal blocks as shown, 
the arched form of these leaves bringing the ends 
into contact with the blocks and keeping a tension 
which opens the breaker as soon as the latch is 



Fig. 301. —Single- Fig. 302. — Single-Pole, Lamin- 
Pole, Knife-Blade ated Contact, I-T-E Circuit 

Contact, I-T-E Cir- Breaker, 

cuit Breaker. 

tripped. This form has an advantage over the 
knife-blade type in that no additional springs are 
required, and there is no danger of sticking. 

Fig. 303 shows a General Electric circuit-breaker 
in which the upper contact is laminated, allowing 
lighter construction of the moving parts of the de¬ 
vice. Connection with the lower contact is through 
a laminated spring. 

Circuit breakers are used for purposes other 
than for protecting the circuit in case of overload. 


296 





Protective Devices 


As an instance, suppose a large motor is running and 
suddenly the power goes off — as when a circuit- 
breaker in the power house opens. The motor slows 
down, but does not stop before the power is on 
again. As long as it is running, the motor acting as 
a generator sends current through its own field, and 
this current, passing through the arm-retaining 
magnet on the starting box, keeps the arm from 


Fia. 303. — Double-Pole, Laminated Contact, General 
Electric Circuit Breaker. 



returning to the “off” position. The power now 
coming on finds the motor almost stopped, but with 
no starting resistance in series with its armature. 
The result is a sudden rush of current. To obviate 
such a condition, motors are sometimes protected 
with a circuit-breaker that will respond to either an 
overload or the failure of line voltage. Figure 304 
illustrates a breaker for this purpose. The fine wire 


297 




Electricity and Electrical Apparatus 

solenoid is connected “across the line,” that is, so 
that it receives the full line voltage, and the latch 
will not stay closed unless this coil is energized. 

Figure 305 shows a circuit-breaker arranged 
for remote control, the motor being operated from 
a distance by a small hand switch when it is desired 
to close the breaker. 



Fig. 304. — Double-Pole I-T-E Overload and No Voltage 
Circuit Breaker. 

A circuit-breaker should have a switch in series 
with it, and when the breaker operates the switch 
should be opened before closing the breaker. Then 
if the short-circuit or overload that caused it to open 
is’still in existence, the breaker is ready to respond 
when the switch is finally closed. The above, how¬ 
ever, does not apply to certain types of circuit- 
breakers which will open even with a hand on the 
handle, though even then it is considered better 
practice to use a switch. 


298 




Protective Devices 



On circuit-breakers designed for high voltage, 
various means are used to interrupt the arc that 
forms when breaker opens. In the “ Magnetic Blow¬ 
out ” type, the current be¬ 
fore arcing across the opened 
contacts must pass through 
a magnet coil. This mag¬ 
net produces a powerful field 
in the space between the 
opened contacts, and the 
reaction between this field 
and the arc current forces 
the arc to one side, and in¬ 
creases its length until it is 
finally ruptured or “blown 
out.” The magnetic blow¬ 
out is adapted to direct- 
current circuits up to 600 
volts. 

On higher potentials, 
switches are used which 
break the circuit under oil. 

A good insulating oil will 
interrupt an arc in a much 
shorter distance than air. 

One of these oil switches, 
when equipped with a trip¬ 
ping coil and arranged to 
open independently of the 
handle, will fill the place 
of both switch and circuit- 
breaker. 

' On very high-potential 
transmission lines the oil 
switches are often placed 
in a separate room, each 
pole of the switch in a fireproof vault by itself, 
and the switch is closed by motors operated from 
switchboard in the main room. The tripping coils 


Fig. 305. — Remote Con¬ 
trol I - T - E Circuit 
Breaker, Motor-Oper¬ 
ated, 10,000 Amperes 
Capacity. 


299 



Electricity and Electrical Apparatus 



Fig. 306. — Condit Type D Oil Switch, Non-Automatic. 
Showing Laminated Contacts. 



Fig. 307. — Condit Type D Automatic Oil Circuit 
Breaker. 

Showing tripping coils, and oil tanks in which contacts are 
immersed. 


300 



Protective Devices 


of these switches are arranged to be operated by a 
hand switch, or by relays arranged to protect the 
circuit from various abnormal conditions, all mount¬ 
ed on the switchboard. Tell-tale lamps show 
whether the oil switch is open or closed. 

Fuses and circuit-breakers serve a purpose simi¬ 
lar to that of the safety valve on a steam boiler. If 
we interfere with the operation of a safety valve — 
weight it or tie it down — it is of no further value in 
preventing excessive pressure and consequent boiler 
explosions. Similarly if the fuses are taken out and 
replaced with larger ones, or with copper, they are 
of no further value in preventing overheating and 
fires. 


301 


CHAPTER XXX 


INTERIOR WIRING 

Underwriters’ Rules — Methods — Materials 

To protect life and property from danger as a 
result of poor workmanship, is the object of the 
National Board of Fire Underwriters in setting forth 
the rules governing electric wiring and apparatus. 
The principle always to be kept in mind is that wires 
must be put up in such a manner that they will not 
readily become crossed or grounded and cause short- 
circuits, with fires resulting from arcing or from 
short-circuit current heating the wires and setting 
fire to wood work, or any combustible material with 
which they may come in contact. 

Some of the more important requirements of 
these rules are touched upon in the following pages, 
together with directions as to the best methods of 
installing wires and fittings to secure the results 
desired. Rubber-covered wire should be used for 
most interior work, though the Underwriters allow 
the use of slow-burning or slow-burning weather¬ 
proof wire in dry, accessible places, if properly put up 
on glass or porcelain insulators. Before beginning 
an installation, the proper sizes of wire should be 
determined, as illustrated in the latter part of Chap¬ 
ter XXXI. 

Wires should not come in contact with other 
wires, whether bare or insulated, nor with water or 
gas pipes or any other conducting material near 
which they may pass. They should be kept from 
such contact by porcelain tubes or circular loom 
through which the wire is passed, and the tubes 


302 


Interior Wiring 


securely fastened by knobs, cleats, or tape, so they 
will not slide away from the place they were intended 
to protect. An example of good overhead wiring 
is given in Fig. 308. 

For ordinary open work, slow-burning weather¬ 
proof wire is allowable. The wires must be securely 
held on porcelain cleats or knobs away from each 



Fig. 308. — Example of Good Overhead Wiring. 


other and from the woodwork or other surface wired 
over, as follows: 


Distance from 

Distance Apart Surface Wired Over 
Less than 300 volts 2 \ inches \ inch 

301 to 550 volts 4 inches I inch 


303 




Electricity and Electrical Apparatus 

Insulators or cleats should not be over five feet 
apart. 

Where wires pass through floors or walls, they 
must be protected by a sleeve of non-absorptive and 
non-combustible material. Porcelain tubes or iron 
pipes are nearly always used, though with iron pipes 
the insulation of the wire must be reinforced. 

In no case should wires be fastened up with 
wire staples, nor should they be fastened to or 
twisted around nails or other conducting materials. 

When wires are run in unfinished lofts, in parti¬ 
tions, or between floors and ceilings, they must be at 
least one inch from any woodwork and if possible 
five inches or more apart. If impossible to keep 



Fig. 309. — Iron Pipe Bushed with Porcelain Tubes, 
for Thick Walls. 

them five inches apart, each wire must be encased 
in a continuous length of circular loom. Wires 
should not be fished for any great distance through 
inaccessible places, and even tor short distances only 
when the inspector can easily see that the rules as 
to spacing, crossing pipes, etc., have been complied 
with. If wires must pass through inaccessible places 
where inspection is impossible, they must be run in 
metal conduit. Rubber-covered wire must be used in 
concealed work of this character, and, except in metal 
conduit, twisted or twin wires must never be used. 

Whenever a joint or splice is made it should be 
mechanically and electrically secure before solder- 


304 







Interior Wiring 


ing, then it must be soldered to prevent deteriora¬ 
tion of the contact through corrosion, after which it 
must be thoroughly taped so that over the joint the 
insulation is as good as at any other place along the 
wire. 

Wood Moulding is often used to conceal wiring 
on ceilings or walls where the appearance of wires 



Fig. 310. — Wires Passing Through Floor. 

Those on left protected by porcelain tubes, those on right by 
iron pipe and circular loom. 


is objectionable. This moulding is made to certain 
standard dimensions, and consists of two parts — 
the base, or moulding proper, with grooves for the 
wires, and the ornamental cap. This construction 
is plainly seen in Fig. 312. The base strip is first 
installed, then the wires are laid in the grooves and 
the cap secured in position with screws. Approved 


305 






































Electricity and Electrical Apparatus 

rubber-covered wire must be used, and this type of 
work is allowed only in dry and accessible places, 
as leakage of current due to moisture might ignite 
the wood. Care must be used to see that nails or 
screws do not pierce the insulation. 

The best method for concealed work is to run 
the wires in Metal Conduit. Standard rubber-cov- 



Fig. 311. — Western Union Splice. 


ered wire may be used if the interior of the conduit 
is lined with a smooth, hard insulating material. 
Uninsulated conduit or plain iron pipe may be used 
if the inner surface is smooth and free from burrs, 
and if made as strong as ordinary gas pipe, but the 
wire used in uninsulated conduit must be rubber- 
covered with an additional braid to take the abrasion 




Fig. 312. — Wood Moulding, Two and Three-Wire. 


incident to drawing in. The inner surface of such 
conduit should be coated or enameled, to prevent 
oxidation, with an enamel that will not be sticky and 
hinder pulling in the wires. The minimum per¬ 
missible inside diameter of conduit is which is 
approximately the inside diameter of “half-inch” 
gas or water pipe. 


306 









Interior Wiring 


Metal conduit must be continuous between out¬ 
let and junction boxes, which must also be of metal. 
Where pipe enters the box, it must be securely 
fastened by lock nuts, and inside the box it must be 
fitted with an approved bushing to protect wire 
from abrasion on the sharp edges of pipe. Conduit 
systems must be completely installed before the 
wires are drawn in, so that subsequent work on the 



Fig. 313. — Galvanized Pipe Conduit, 
with Insulating Lining. 


conduits will not injure the wires. Before current 
is thrown on, the conduit system must be thoroughly 
grounded to water pipes or to the frame of the build¬ 
ing. 

On account of the neat appearance and the 
protection afforded from mechanical injury, conduit 
systems are often used instead of open wiring. The 



Fig. 314. — Enameled Pipe Conduit, 
with Insulating Lining. 


conduits are put up like steam or water pipes, and 
look like open plumbing. Figs. 316 to 321 show 
a line of outlet boxes designed for use with such a 
system, where neat appearance is desirable. These 
outlets are used like pipe fittings — T's, elbows, etc. 
— being made of cast iron, with hubs tapped for 
standard pipe threads, and arranged to protect the 
wires from sharp edges of the pipe. Bushings and 


307 







Electricity and Electrical Apparatus 

lock nuts are unnecessary. Fig. 321 represents one 
of these fittings designed for a junction box, ar¬ 
ranged to contain a fuse cut-out and snap switches 
to control the branch circuits. 

When Fixtures are supported from the gas pip¬ 
ing, an approved insulating joint, similar to that 
shown in Fig. 322, must be installed as close as 
possible to the ceiling, and wires must be insured 
against possible contact with the pipes above this 
joint by porcelain tubes or circular loom. 

In combination gas and electric fixtures, suffi¬ 
cient space must be allowed between gas pipe and 



Fig. 315. — Conduit Outlet Box, with Crouse-Hinds 
Fixture Rosette. 

Showing conduit with bushing inside and lock-nut outside of box. 

outside casing to prevent jamming the wires. Be¬ 
fore connecting to the lighting circuit, fixtures should 
be tested for grounds or short circuits. Number 
18 B. & S. rubber-covered, or under certain condi¬ 
tions, slow-burning wire, is allowable in fixture 
work, but No. 16 is advised for mechanical leasons. 

Drop lights are usually hung by flexible lamp 
cord from Rosettes mounted on the ceiling and con¬ 
nected to the circuit wires. Several designs are 
shown in the accompanying illustrations. The cir¬ 
cuit wires on each side are attached to binding 
plates and screws in the base of rosette. The lamp 


308 


Interior Wiring 


cord is attached to other binding screws in the cap. 
A knot should be tied in the cord where it passes 
through hole in center of rosette, so weight of lamp 



Fig. 316. — Crouse-Hinds 
Condulet, Type A, with 
Two-Wire Cover. 



Fig. 317. — Type B Con¬ 
dulet, with Three-Wire 
Cover. 


will not pull wire away from binding screws. In a 
fused rosette there are four brass contact punchings 



Fig. 318. — Type C Condu- Fig. 319. — Type U Con- 
let, with Metal Nipple dulet, with Porcelain 

Cover. Nipple Cover. 


in the cap, two on each side. A piece of small luse 
wire is connected between the contacts on either 



Fig. 320. — Type X Con- Fig. 321. — Type ZX Condu- 

DULET, WITH PLAIN MeTAL LET, FOR CUTOUTS AND SNAP 

Cover. Used as a Junc- Switches on Branch Cir- 
TION BOX. CUITS. 

side, the lamp cord being attached to one, and the 
other is connected by means of a screw to the bind¬ 
ing plate in the base. 


309 




Electricity \nd Electrical Apparatus 

# Rosettes should be made of non¬ 

combustible; non-absorptive insula¬ 
ting material, and should have a base 
high enough to keep the wires at least 
i inch from the surface to which it is 
attached. Rosettes should be fused 
Fig. 322 . for 3 amperes, or if fuseless rosettes 
Insulating Joint. are use d, not more than 660 watts 

HARVEY IIUBBELL, ^ p bg 

nected to one fused circuit, as lamp 
sockets and the ordinary flexible lamp cord are 
especially liable to short circuits. Where the insu- 



Note that an insulating ring is used between the condulet and 
canopy, as otherwise the insulating joint would be useless. 


lation is cut away from the line wire to connect it 
to the rosette, care must be used not to cut or 
injure the wire so as to cause it to break. 


310 







Interior Wiring 


Flexible Cord is made up of several copper-wire 
strands, between the sizes of 26 and 30 B. & S. gauge, 



Fusible, for One-Piece Two-Piece 

Cleat Wiring. Fuseless. Fuseless. 


Bryant “Junior” Rosettes. 




Fig. 327. — Crouse-Hinds Cleat Rosette. 



Fig. 328. — “Conduletto” Rosette. Crouse-Hinds Co. 

twisted together. The total cross-sectional area 
of the strands should not be less than that of a No. 


311 







Electricity and Electrical Apparatus 


18 B. & S. gauge wire. Rubber insulation is used, 
and instead of tinning the wires, they are held to¬ 
gether and protected from the action of sulphur in 




Fig. 329. — Brass Lamp 
Socket and Parts. 
Bryant Electric Co. 



Fig. 330. — Weatherproof 
Lamp Socket. 
Bryant Electric Co. 



Fig. 331. 

Hubbell Pull Socket. 




Interior Wiring 


the rubber by a winding of cotton thread. Over 
the rubber insulation an ornamental braid is used, 
not impregnated with any weather-proofing com¬ 
pound. This cord is intended for supporting one 
or two lamps only, or for use on portable lamps, very 



Fig. 333. — Crouse-Hinds Temporary Socket. 

For temporary illuminations. Insulation need not be removed 
from wires, as pointed screws pierce the insulation. 


small motors, or other small apparatus. It should 
not be used to support clusters. With metal lamp 
sockets, insulating bushings should be used to pro¬ 
tect the cord where it enters the socket. 

Lamp Sockets should be so constructed that the 
inside of the metal shell, if metal is used for the ex- 



Fig. 334. — Crouse-Hinds Cleat Receptacle. 

terior, shall be thoroughly insulated from any wires 
attached to the inner part of the socket. Care 
should be taken in attaching wires to the rosette and 
socket to see that no ends are left to fray out and 
cause short circuits. All strands should be securely 
fastened under the screws. 


313 



Electricity and Electrical Apparatus 

Service wires entering a building should be con¬ 
nected to a fuse cut-out or circuit-breaker as near 
as possible to the point of entrance, so as to protect 




Fig. 335. — Crouse-Hinds Moulding Receptacle. 

all wiring in the building. Fig. 336 illustrates a cut¬ 
out box designed to be mounted on the outside of 
the building or on a pole, from which the service 
wires are brought into the basement through 
conduit. 



Fig. 336. — Crouse-Hinds Condulet, Type FF. Service 
Entrance Cutout Box. 


314 



Interior Wiring 


Next after the fuses, and as near them as possi¬ 
ble, is placed a switch, to control the entire build¬ 
ing. Knife switches should be supported on non- 



Fig. 337. — General Electric Knife Switch. 



Fig. 338. —Crouse-Hinds Type B, Face Connection Knife 
Switch, Arranger for Enclosed Fuses. 



Fig. 339. — Crouse-Hinds Type B, Back-Connection, 
High-Capacity Knife Switch. 

combustible, non-absorptive insulating material 
such as slate or porcelain. Hinges should be equip¬ 
ped with spring washers held by nuts or pins so that 


315 









Electricity and Electrical Apparatus 



Fig. 340. — Crouse-Hinds Three-Wire Service Switch, 
Arranged for Enclosed Fuses, in Iron Cabinet. 



Fig. 341. — Perkins Fig. 342. — Perkins Double-Pole 
Snap Switch. Snap Switch. Showing Mechanism. 




Fig. 343. — Indicating Fig. 344. — Perkins Flush 

Snap Switch, Mounted Push Button Snap Switch, 

on Type G Condulet. 


316 









Interior Wiring 



a secure, firm connection is maintained. Arcing at 
this point would soon burn the switch so as to render 
it useless. Attention is called to the excellent 
design of spring washer on the switches in Figs. 338 


Fig. 345. —Unlined Cab- Fig. 346. — Slate-Lined Cab¬ 
inet, with Snap Switches inet, with Knife Switches 
and Plug Fuses. and Open-Link Fuses. 


and 339. They are made of spring steel, cup-shaped, 
tempered and copper-plated. 

To work at low temperature rise, the average 
full-load current in a switch should not exceed 1,000 


317 









Electricity and Electrical Apparatus 

amperes per square inch of cross-section, or 50 to 
75 amperes per square inch of sliding-contact area. 
It should be made heavy enough in any case, even 
for small capacities, to withstand considerable me¬ 
chanical abuse. The voltage and current capacity 
should be plainly stamped on each switch. Knife 
switches should be mounted with the handle up so 
that they will not tend to close by gravity when open. 



Fig. 347. Fig. 348. 

Panel Board, and Iron Cabinet with Wiring Gutter 
and Wood Trim. Crouse-Hinds Co. 


Switches and fuses should be placed on all ser¬ 
vice wires, whether overhead or underground, and 
should always be accessible. Wherever in the build¬ 
ing branches are taken off, using smaller wire than 
the mains, fuses must be installed of a capacity not 
greater than that of the smaller wire, to protect it. 

Snap switches should indicate whether current 
is “on” or “off” to prevent mistakes or accidents. 
They should be marked with the voltage and cur¬ 
rent capacity, and should be large enough not to 


318 















Interior b Wiring 

heat excessively when carrying their rated current. 
Inside of metal cover should be lined with fire-proof 
insulation. 

A very good and reliable scheme is to have all 
switches and fuses enclosed in an iron box, or a wood 
cabinet lined with iron, slate, or asbestos, or if snap 
switches and enclosed fuses are used, an unlined 
wooden cabinet is permissible. The panel board 




Fig. 349. — Metering Panel and Cabinet. 
Crouse-Hinds Co. 


and cabinet in Figures 347 and 348 make a very 
neat, convenient, and attractive arrangement, espe¬ 
cially if the door is built with a glass panel. Service 
is-brought to the main fuses, thence to main switch, 
then to the bus bars, from which it is distributed to 
the several branch-circuit switches and fuses. With 
fuseless rosettes and not over 12 lights on each branch 


319 















Electricity and Electrical Apparatus 

circuit, all fuses and switches are kept in one place 
— an advantage readily appreciated when the lights 
go out. 

Fig. 349 shpws a meter panel and cabinet espe¬ 
cially adapted to office buildings. When two or more 
rooms are rented to one tenant, the lights in all of 
his rooms may be connected to the same meter by 
changing a few screws in this panel. 


320 


CHAPTER XXXI 
EXTERIOR WIRING 

Underwriters’ Rules — Transmission Lines 

Outside wiring is fully as important as inside 
wiring, and in many cases more important. In the 
equipment of commercial power and lighting plants, 
it represents a large part of the investment, some¬ 
times cpsting more than the complete generating 
station — generators, engines, boilers, building and 
all. Usually the voltage is much higher than is 
allowed for interior work, increasing the danger to 
life and property. There are many things to con¬ 
tend against from which interior wiring is largely 
protected. 

It is important that exterior wiring be put up 
so it will not inflict damage on itself or other prop¬ 
erty, in case of any accident to which it is liable, 
and so it will not interfere with prompt action in 
emergencies involving danger to other property. 

Near buildings, wires must be so placed as not 
to interfere with fighting fire, or endanger the lives 
of firemen, in case fire breaks out in the building. 
Before work is started, definite plans should be laid 
out. Too often this is not done, the lineman run¬ 
ning his wires the easiest way, regardless of looks or 
safety. 

Service wires from the line to outside of build¬ 
ing may be of weatherproof insulation, but from 
there through the wall to main cut-out and service 
switch, it must be rubber-covered. Where wires 
enter the building the holes must be bushed with 
porcelain tubes or the equivalent, slanting upward 


321 


Electricity and Electrical Apparatus 

toward the inside so water will not follow in along 
the wire. Outside, the wires must be attached to 
insulators, and between insulator and outer end of 
tube each wire must have a “drip loop” as shown 
in Fig. 350. Rain water will thus collect and drop 
from the lowest point of this loop,’ and not tend to 
follow the w T ire into the building. 

On low potential systems the wires may be 
brought into the building through a single iron con¬ 
duit, provided that the outside end curves down¬ 
ward and is carefully sealed, or is equipped with an 
approved service head, to prevent the entrance of 
moisture. The outer end must be at least one foot 



Fig. 350 . — Entrance Bushing and Drip Loop. 

from any woodwork and the inner end must extend 
to the service cut-out (Fig. 351). 

Out of doors, the wires must be at least 12 
inches apart, supported on petticoat insulators of 
very good insulating material such as glass or porce¬ 
lain, and so placed that moisture cannot form a 
cross connection between the wires. If wooden 
blocks or pins are used to support the insulators, 
they should be given two coats of waterproof paint. 
Petticoat insulators (Fig. 352) are required, as in 
rainy weather they will nearly always have some 
dry surface between wire and pin to prevent leakage. 

Figs. 353 and 354 show wires running over roofs. 
They must be at least seven feet above the highest 


322 









Exterior Wiring 


point of a flat roof and one foot above the ridge of 
a pitched roof, whether attached or not. The sup¬ 
porting structures should be well made, of strong 
material, so as not to allow wires to fall on the roof 
or sag so as to^come in contact with persons walking 

In making splices, the 
same rules apply as on in¬ 
terior wiring. The joint 
must be electrically and 
mechanically good before 
soldering, and after solder 
is applied it must be taped 
until the insulation is as 
good as elsewhere on the 
wire. 

In tying wires to the 
insulators, care should be 
used not to put a sharp bend 
in the line wire, especially 
if it is hard-drawn copper. 
The simple U-shaped tie- 
wire plan shown in Fig. 355 
is objectionable on this ac¬ 
count. A tie made like Fig. 
356 will, if properly made, 
hold the line-wire firmly to 
the insulator, without bend¬ 
ing it. 

Fig. 351. —Conduit For most high-tension 

Service Entrance. transmission lines, porcelain 
insulators are made with a 
groove across the top for the line wire, and another 
groove for the tie wire just below and around 
the top. The tie is made as shown in Fig. 357. 
These porcelain insulators, when used for voltages 
over 20,000 or 30,000, are made in two or three 
parts, which are held together with a vitreous 
cement. 


on the roof. 



323 


















Electricity and Electrical Apparatus 


Electric light and power 
wires should not be placed on the 
same cross arm with telephone or 
telegraph wires, and when neces¬ 
sary to run them on the same 
poles, every precaution should be 
taken to avoid the possibility of 
the wires of one circuit coming in 
contact with those of the other. 
Such crossing of circuits would 
endanger lives of telephone or 
telegraph operators, as well as 
being a possible cause of fire in 
the buildings which such lines 
enter. The two inside pins on the cross arms carry¬ 
ing light or power wires should be at least 26 inches 



Fig. 352. 

Double Petticoat 
Glass Insulator. 
Pettingell- 
Andrews Co. 



Fig. 353. — Substantial Wooden Roof Structure. 


apart, so a lineman may safely climb between the 
wires to reach the upper cross arms. 

When the line is to operate at high potential, 
extreme precautions should be taken, on account of 
the danger likely to result from failure of any part 
of the structure. It must be erected so that no 
accident or conceivable combination of accidents 
would bring any of its wires in contact with other 
electric circuits. 

If run near existing lines, it should not be near¬ 
er than the height of the taller pole line. This will 


324 





Exterior Wiring 


prevent any crossing of wires should the taller pole 
break near the ground and fall toward the lower line. 
Where high-potential wires must unavoidably 



Fig. 354. — Iron Pipe Roof Structure. 



Fig. 355. Fig. 356. 

Tying Wire to Insulator. 
Objectionable Method. Good Method. 



Fig. 357. — Method of Tying Cable to Insulator 
with Grooved Top. 

come nearer than specified above to other circuits, 
or cross over them, or where they must be carried 


325 




Electricity and Electrical Apparatus 

on the same poles, extra precautions must be taken 
to reduce to a minimum the liability of a breakdown, 
and to avoid the dangerous effects of a break should 



Fig. 358. — Three Porcelain High-Tension Insulators. 


Pettingell-Andrews Co. 



Fig. 359. — High-Pressure Line Crossing Other Lines. 

it occur. When on the same poles, the high-poten¬ 
tial should be at least three feet, preferably five, 
above the low-potential wires. This lessens the 


326 












Exterior Wiring 


danger to linemen working on the 
low-potential circuits. 

Fig. 359 shows a high-tension 
line crossing another line. The 
height and length of the cross-over 
span is made such that the shortest 
distance between the lower cross 
arms of the upper line and any wire 
of the lower line will be greater than 
the length of the cross-over span. 
With this arrangement, the wires 
could not touch each other, unless the tallest pole 
broke, even if a wire were to part at one of the 



Fig. 360 . 
End-Insulator 
Guard. 



insulators. The outer wires of the upper line should 
be guarded, as shown in Fig. 360, to prevent it drop¬ 
ping over the end of cross arm on to the lower line 
in case its insulator should break or if its tie wire 
should become loosened. 


327 






Electricity and Electrical Apparatus 

Other methods of making crossings are illus¬ 
trated in Figs. 361 and 362. The joint-pole crossing, 
while not as good as the first method, may be adopt¬ 
ed if the other is impracticable. If neither method 
is feasible, the screen protection may be used. The 
screen should be supported on high tension insu¬ 
lators, or it should be thoroughly grounded and of 
such heavy construction that it will carry to ground 
any current a falling high-tension wire can deliver 
to it on short circuit. 



Fig. 362. — Crossing Protected by Screen. 

On the joint-pole crossing, four guard wires 
(shown heavier than the others) extend for one span 
either side of the joint pole parallel to the low-tension 
wires, and protect them from contact with broken 
wires of the upper circuit. These guard wires are 
on high-tension insulators. The minimum distance 
between high and low-tension wires should be three 
feet. Five is better. The end guards, to prevent 
wires slipping off ends of cross arms and dropping 
on the lower wires, should extend about six inches 
above the level of transmission line. 


328 












Exterior Wiring 


Lightning arresters must be placed on all over¬ 
head wires which connect with a station. A light¬ 
ning arrester is a device which will allow a moment¬ 
ary excessive voltage to discharge to ground across 
an air gap or other barrier, but will not allow current 






a number of places to * s* ' f 
whole ^ -j”-* 

'Zf'. 

r 110.11 

abou 

, • P' rn . „ 


Ground wre riveted_ __ 

copper plate, and soldered for while distance across > . , 

--- ■*./" .■.*/{</ 

i N 0 .I 6 gage copper plate 
[> about3x6feet,atle»el of 
; permanently damp earth 


Fig. 363. — Lightning Arrester House, with Ground 
Connection. 


from the line to follow the arc so established. They 
should be placed as near as possible to the point 
where wires enter the building, and in an easily 
accessible place away from combustible material. 
Kinks and sharp bends in the wire running from the 
outdoor lines to the arresters and from arresters to 


329 

























Electricity and Electrical Apparatus 

ground should bo avoided as far as possible, as they 
may offer a high resistance to lightning current, 
which is of very high frequency, and cause it to dis¬ 
charge at some point where considerable damage 
might be done. 

Lightning arresters must be connected to ground 
with a copper .wire No. 6 B. & 
S. or larger. Gas pipes within a 
building must not be used for a 
ground connection. A choke coil 
is sometimes introduced in the 
circuit between arrester and gen¬ 
erator. This acts like a dam to 
the lightning discharge, and the 
overflow is through the arrester 
to ground. Fig. 363 shows a very 
good arrangement for a ' power 
house where several arresters are 
to be used. 

It is recommended that ar¬ 
resters be placed at intervals 
along the entire system, especially 
if it is a long line through the 
country. The ground connec¬ 
tions may be made with a one- 
inch galvanized iron pipe driven 
about 8 feet or until it reaches 
permanently moist earth and ex- 
Arrester. tending at least 7 feet above 

ground. The ground wire should 
be securely soldered to a brass plug firmly screwed 
into the pipe, and both strongly stapled to the pole 
so there will be little danger of the connection 

being broken. urn 

A good ground is very important, as the efn- 
ciency of the protection would be greatly impaired 
if the ground connection were poor. Wherever the 
earth is dry and a good ground cannot surely be 
obtained, an excavation 4 or 5 feet deep should be 



Fig. 364. 


330 



Exterior Wiring 


made, and after placing the copper ground plate or 
iron pipe in the hole, it should be filled with crushed 
coke or charcoal about pea size. This improves the 
electrical connection between pipe or plate and 
earth. 

When running a transmission line through the 
country, the poles should be as nearly as possible 
the same size, except where other sizes are necessary, 
as at points where it crosses other lines. This gives 
the line a neat appearance. Poles should be set in 
ordinary firm ground to the following minimum 
depths: 


Feet 


Feet] 

25-ft. Poles, 5 

55-ft. Poles, 

7i 

30-ft. Poles, 5^ 

60-ft. Poles, 

8 

35-ft. Poles, 5£ 

65-ft. Poles, 

Si 

40-ft. Poles, 6 

70-ft. Poles, 

9 

45-ft. Poles, 6^ 

75-ft. Poles, 

9^ 

50-ft. Poles, 7 

80-ft. Poles, 

10 


In solid rock they may be set two feet less, and in 
soft, marshy ground, or ground that is likely to be¬ 
come so at any time, they must be set to a greater 
depth, depending on conditions. 

Tops of poles should be made wedge shape and 
painted with weatherproof paint so that they will 
shed rain and snow. The bottom of this wedge 
should be about four inches above the top of the 
upper gain, or place cut into the pole to receive the 
cross arm. The wedge should be in a line parallel 
with the wires. 

Cedar and chestnut are used most extensively 
for pole timber, though juniper, pine, locust, cypress, 
catalpa, and oak are used to some extent, principally 
where it grows in abundance near the locality. 

Cross arms are made from yellow pine, Oregon 
or Washington fir, cedar, cypress, or white pine. 
Wood insulator pins are nearly always made from 
locust. For voltages above 20,000, metal pins should 


331 


Electricity and Electrical Apparatus 

be used, as wood is liable to become carbonized from 
static leakage. 

The life of poles ranges from 5 to 35 years, de¬ 
pending on 

1. The kind of wood. 

2. The character of the wood as regards 

A. Heart or sap wood. 

B. Seasoned or green. 

C. Where grown and when cut. 

3. The amount of combined air and mois¬ 
ture to which pole is exposed. 

It appears that a combination of oxygen of the 
air and moisture is needed to promote growth of the 
destructive fungi. This is why poles invariably 
decay most rapidly just below the ground line. 

Poles cut in winter, when the sap is down, have 
the longest life. The poles should be stripped of 
bark and seasoned under cover for several months. 
It has been found that green timber placed under 
running water for a month and then dried under 
cover, seasons more rapidly than if it had not been 
placed in water. It appears to wash out the sap and 
leave clear water in its place, which dries out more 
quickly. 

Many processes have been patented for treating 
timbers to prevent or retard decay. The most im¬ 
portant of these processes is that of impregnating 
with creosote oil. The green pole is placed in a 
cylinder and baked in live steam, then subjected to 
a vacuum for four to eight hours, or until no mois¬ 
ture comes from the cylinder, then creosote oil is 
forced into the cylinder under pressure until the pole 
has absorbed the required amount. 

Concrete poles are being used to some extent, 
and, especially for high-tension transmission, towers 
of structural steel. The cost is greater than for wood 
poles, but longer spans are used and so fewer towers 


332 


Exterior Wiring 


are needed. Also concrete and steel have much 
longer life than wood poles. They are not affected 
to such an extent by climatic and weather condi¬ 
tions, nor by insects. 

Wood poles carrying wires from No. 6 to No. 1 
should be spaced about 150 feet apart, or 35 poles 
per mile. For heavier wires, they should be spaced 
closer, up to 48 per mile for 6 No. 0000 wires, and 
66 per mile for 6 250,000 C.M. cables. Steel towers 
are usually spaced from 300 to 500 feet apart, or 18 
to 10 per mile. 

Poles should be numbered consecutively from a 
certain definite terminal or junction, and the number 
should be painted clearly upon each pole with white 
lead at a distance of about 6 feet above the ground. 
In this way a record may be kept of the life of every 
pole, and the line is provided with innumerable ref¬ 
erence points which are valuable in directing re¬ 
pairs, alterations, etc. 

All poles on curves should be well anchored, as 
the tension on the line wires would tend to pull them 
over. The cross, arms on every other pole should be 
placed on one side of the pole, and those on the 
alternate poles on the other, or in other words the 
cross arms on adjacent poles should face each other. 

The size of wire to use for a transmission line 
depends on several considerations. It must be 
large enough to be mechanically strong. On this 
account no wire smaller than No. 6 B. & S. should 
be used on pole lines. It should be large enough to 
carry the current without undue heating. This is 
determined from the wire tables. It should be large 
enough so that its resistance will not waste an un¬ 
due amount of energy in PR losses, nor cause poor 
regulation of voltage on account of large IR drop 
at full load. That we may more fully grasp the 
meaning of these two latter requirements, let us 
work out the proper size of wire to be used in the 
following example. 


333 


Electricity and Electrical Apparatus 

Suppose we wish to transmit current from a 
power plant to supply sixty 110-volt incandescent 
lamps in a residence a half mile distant. The cur¬ 
rent required, roughly estimated at \ amperes for 
each lamp, would be 30 amps. 

From the wire tables we find that a No. 10 
weatherproof wire would carry the current without 
undue heating, but from considerations of strength 
we must use No. 6. Since the current must go out 
over one wire and return over the other, it must flow 
through one mile of wire, the resistance of which 
will be 5.28 times that given in the tables for 1000 
feet. Looking this up in the table we find the total 
resistance will be 5.28 X .3953 = 2.087 ohms. 

With 30 amperes flowing the IR drop in the 
line would be 30 X 2.087 = 62.61 volts, and the 
I 2 R loss would be 30 X 30 X 2.087 = 1878.3 watts. 
If the lamps are burned an average of three and one- 
third hours a night for 300 nights in the year, or a 
total of 1000 hours, and power costs 10c. per KW.- 
hour, in a year this will waste 

1878.3 X X $0.10 = $187.83 worth of power. 

Suppose the interest on money invested, and 
the taxes and depreciation in value of the copper 
wire were 10% per year. At 25c. a pound, one mile 
of No. 6 wire costs 5.28 X 112 X $0.25 = $147.84. 
Its cost for one year is 10% of $147.84 or $14.78. 
We see that the power wasted costs more than 12 
times as much as the expenses on the wire. 

Let us try again, using larger wire, say No. 2: 

Resistance = 5.28 X .156 = .825 ohm 
IR = 30 X .825 = 24.75 volts 
PR = 30 X 30 X .825 = 742.5 watts 
Cost of wasted power per year = 

742.5 X ^ x $o.io = $ 74.25 


334 




Exterior Wiring 


Cost of wire per year = 

5.28 X 268 X $0.25 X .10 = $35.40 
Or with No. 1 wire, 

Resistance = 5.28 X .124 = .655 ohm 
IR = 30 X .655 = 19.6 volts 
PR = 30 X 30 X .655 = 589 watts 

Cost of wasted power per year = 

589 X ^ x $o.lO = $58.90 

Cost of wire per year = 

5.28 X 306 X $0.25 X .10 = $40.40 
If we use No. 0, 

Resistance = 5.28 X .0983 = .519 ohm 
IR = 30 X .519 = 15.6 volts 
PR = 30 X 30 X .519 = 468 watts 
Cost of power per year = 

468 X x $0.10 = $46.80 

Cost of wire per year = 

5.28 X 400 X .25 X .10 = $52.80 


Size Wire 

Line Drop in 

Volts 

Voltage Drop, 
in Per Cent, of 
Generator Voltage 

Cost of Wasted 
Power per Year 

Cost of Wire per 
Year 

Total 

No. 6 

62.6 

36.3 

$187.83 

$14.78 

$202.61 

No. 2 

24.7 

18.3 

74.25 

35.40 

109.65 

No. 1 

19.6 

15.1 

58.90 

40.40 

99.30 

No. 0 

15.6 

12.4 

46.80 

52.80 

99.60 


From the above we can see that under the con¬ 
ditions given, it would be most economical to use 
the No. 1 wire, but the voltage drop, 19.6 volts, 


335 












Electricity and Electrical Apparatus 

would be excessive, as it would be nearly 18% of the 
voltage required at the lamps. To have fairly good 
regulation of voltage at the lamps, the line drop 
should not exceed 5%, which in this case would be 
5.5 volts. To effect this, the resistance must be less 

than ^ or .1833 ohm, or = -0347 ohm per 

30 o.2o 

1000 feet. Referring to the wire tables, we find that 
a 300,000 C.M. cable is the smallest that will fulfill 
this requirement. 

The following will be of assistance in forming 
an approximate idea of the cost of a transmission 
line, though obviously it is impossible to produce 
close general figures when labor varies from $1.75 
to $3.50 per day, values of material fluctuate widely, 
and line construction may be carried on in summer 
or in winter weather. 

The right of way will cost from nothing to sev¬ 
eral hundred dollars per mile. 

Clearing the right of way will cost from the value 
of the wood taken off to $75.00 per acre or more. 

Holes will cost from 25c. each in sandy soil to 
$1.75 or more each in hard clay or frozen ground. 

For a single 3-phase 6600-volt circuit, taken as 
an example, the cost per mile should be about as 
follows: 


Labor 

Digging, at 50c. per hole.$ 20.00 

Fitting, at 25c. per pole. 10.00 

Erecting, at 30c. per pole. 12.00 

Distributing, at 35c. per pole. 14.00 

Stringing 3 Conductors... 30.00 


$ 86.00 

15% contingencies. 12.60 


336 


$98.60 











Exterior Wiring 


Materials 

Poles, 35 ft. — 40 per mile.$240.00 

Cross Arms . 4.50 

Braces . 3.50 

Line Hardware. 17.00 

Guy Wire, 1000 ft. f", 7-strand.. 10.00 

Pins . 15.00 

Insulators . 20.40 


$310.40 

5% for extras. 16.00 


$326.40 

Material.$326.40 

Labor. 98.60 


Total .$425.00 


To the above must be added the cost of 3 miles of 
wire of the size used, the cost of obtaining and clear¬ 
ing the right of way, and the freight on materials 
used. 

Poles of southern cedar or juniper cost about 
$3.25 for 25 ft. poles, $6.00 for 35 ft., $17.50 for 50 ft., 
and $28.00 for 60 ft. To this must be added the 
freight from the shipping point near the forests 
where the timber grew. Chestnut poles cost less, 
35ft. poles being sold for $4.50 to $5.00 each. Stand¬ 
ard Si" X 4i" yellow pine cross arms cost about 3Jc. 
per lineal foot. Larger sizes are special and cost 
more, 4" X 6 ", for instance, costing about 8 cents 
per lineal foot. 

Line hardware consists of such materials as 
bolts, lag screws, washers, guy anchors, guy clamps, 
eye bolts, thimbles, guard irons, cross-arm braces, 
etc. These should be galvanized to prevent rusting, 
as they are exposed to all kinds of weather. 


337 
















Electricity and Electrical Apparatus 

Cross arms are placed in the gains previously 
cut for them and bolted to the poles with two bolts 
placed diagonally. Then diagonal iron braces are 
applied, being attached to the cross arm at one end 
and to the pole at the other with lag screws. 

Guy wire, being usually made up of seven strands 
of galvanized steel wire, is difficult to tie or splice, 
as can be done with copper wire. The usual method 
of attaching is to pass the cable through an eye, or 
wrap it two or three times around the pole, and, 
bringing the end back by the side of the main guy, 
clamp them together with bolts and malleable cast¬ 
ings designed for the purpose. Where the wire 
passes through an eye, a thimble should be used to 
prevent too sharp a bend in the wire. 


338 


CHAPTER XXXII. 

CENTRAL STATIONS 

Operation — Equipment — Switchboards — Storage 
Batteries — Their Uses. 

The function of the central station is to supply 
its customers with electricity, much as the gas and 
water works furnish gas or water to consumers. 

An electrical power plant, whether private or 
public, for pleasure or profit, should be prepared to 
furnish current whenever there is a demand. In the 
case of a central station, supplying hundreds or 
thousands of customers, with varied requirements, 
it is even more imperative that nothing interfere 
with the continuous operation of the station. Hence 
reliability is the foremost requirement of the appa¬ 
ratus comprising the equipment. 

In a commercial plant it is of vital importance 
that the apparatus be of a type to insure economical 
and profitable production, enabling consumers to 
obtain current at rates not only reasonable, but low 
enough to make electric lights preferable to gas and 
other illuminants, and electric motors better and 
cheaper for power than gas or steam engines. 

A power plant and its equipment must be de¬ 
signed and selected to suit the conditions which it 
must meet. The conditions vary in different 
localities. For a plant of any size or importance the 
services of a competent engineer should be secured. 

In laying out a plant, provision should be made 
for future growth. Many plants have been designed 
with no regard for this, and additions are often diffi¬ 
cult and expensive. Not only are ground space and 


339 


Electricity and Electrical Apparatus 

future building room to be considered, but likewise 
the type of units and the system adopted should be 
such that new equipment can be selected to readily 
harmonize with the old. 

There are three general classes of power plant: 

1. Hydraulic, in which the source of energy 
is a waterfall or a considerable quantity of water 
flowing from a higher to a lower level through water 
wheels or turbines. 

2. Steam, usually generated by burning coal 
under boilers. 

3. Gas, natural or artificial, the latter pro¬ 
duced from coal in retorts or generators, or in gas 
producers. Many small engines use gas produced 
from the more volatile constituents of petroleum, 
as gasoline. Some large steel plant installations use 
the gas from the blast furnaces. 

The Location, so far as other conditions permit, 
should be near the center of the load, so as to econo¬ 
mize in the use of copper and in the amount of 
energy wasted in transmission. This point is often 
overruled by other considerations. If a hydraulic 
plant, the location is fixed. When the waterfall is 
at a considerable distance from the load center, the 
power is often transmitted at high tension from the 
power plant at the falls to a sub-station located near 
the load center. Here it is transformed and dis¬ 
tributed at low voltage to the consumers. 

If a steam plant, it must be located where water 
is obtainable in quantities and of a quality suitable 
for feed water for the boilers and for cooling water 
for the condensers. Railway or waterway facili¬ 
ties should be available, so that the coal or other 
fuel used can be delivered at the least expense. 

In coal regions, the plant is sometimes located 
at a coal mine. In this way the expense of trans¬ 
portation is saved, it being less expensive up to cer- 


340 


Central Stations 


tain distances to transmit the power as electric cur¬ 
rent over wires than as coal in cars. Also coal may 
be used of a quality so poor that it would not be 
worth the expense of transportation away from the 
mine. 

Other points affecting the location are: relative 
cost of land; suitability of the land as regards foun¬ 
dations, etc.; proximity to churches, schools, or 
other places where noise or smoke are prohibitive; 
and liability to legal complications. 

In selecting the Equipment, we must consider 
what loads the machines will probably have to 



Fig. 365. — Central Station Load Curve. 


carry, and for how long. Even with a large day 
load, of motors in factories, and special devices, such 
as washing machines and flat irons, in the homes, 
the heaviest or “peak** load comes at night, from 
about six, to eight or nine, in the ordinary central 
station. 

Fig. 365 shows a typical daily Load Curve. To 
carry the peak of this load requires a maximum out¬ 
put of about 1000 K.W. It would be very costly 
to operate one set of 1000-K.W. capacity 24 hours 
a day for such a load curve. In a plant like this, 
if the equipment consisted of three 300-K.W. genera- 


341 













































Electricity and Electrical Apparatus 

ting units, one set could handle the load from a little 
before midnight until the next afternoon, the second 
set being started at about 5 P.M. At 6.30 P.M., it 
would be necessary to throw in the third set, which 
could be shut down again at 8.30. At 7.30 the 
machines would be carrying about 12% overload. 
One of the three sets would not be operated very 
much. Thus not only are expenses reduced, but 
also, if one unit is temporarily disabled, the plant is 
not shut down. The period from midnight until 
5 P.M. would suffice to make ordinary repairs, or in 
an emergency the two other units could handle the 
peak, the duration of which is about 2J hours, on 
their overload capacity, if carefully watched. 

In some cases an additional set is installed. 
This not only gives a larger factor of reliability, but 
also permits the overhauling of any set at any time 
— making it possible to keep the apparatus in the 
best condition. However, with the three similar 
units, if a spare armature, set of field coils, and minor 
repair parts were kept on hand, reasonable reli¬ 
ability would be insured, as in almost any emergency 
repairs could be quickly made to any machine. 

Let us now take up the different types of gene¬ 
rators, and the ways in which they are connected 
together when one is not enough to carry the load. 

They are not often connected in series, except 
as boosters in railway work, or for three-wire systems 
or for series arc lighting or similar purposes where 
high voltage is desired. They are, however, fre¬ 
quently operated in multiple. 

With two shunt machines operating in multiple 
(Fig. 366), suppose the voltage of machine No. 2 is 
slightly higher than No. 1. No. 2 would tend to 
force current through No. 1 and make it operate as 
a motor, but this increase in load on No. 2 lowers 
its voltage. The tendency of two shunt machines 
in multiple is to distribute the load evenly and 
operate satisfactorily. 


342 


Central Stations 


Series-wound machines can be operated in 
series, but not in parallel without special equalizer 
connections. Referring to Fig. 367, if machine No. 1 
speeds up a little or for any reason begins to gene- 



Fig. 366. — Two Shunt-Wound Generators in Multiple. 

rate a little higher voltage than No. 2, it would at 
once commence to take more than its share of the 
load. With this increased current, the field would 
be strengthened, and cause a new increase in the 



Fig. 367. — Two Series-Wound Generators in Multiple. 

voltage, still further aggravating the unequal dis¬ 
tribution of load. 

To overcome this action, an equalizing con¬ 
nection must be made as in Fig. 368. Now if No. 1 
speeds up and generates a higher voltage than No. 2, 
some of the current, after passing through the arma- 


343 



















Electricity and Electrical Apparatus 


ture, will pass down the equalizer and through field 
of No. 2. In other words, the currents from both 
armatures, whether equal or not, will divide equally 
before passing through the field windings, if resist¬ 
ances of the latter are equal and that of the equalizer 
negligible. 

Compound machines, combining as they do 
the characteristics of both shunt and series machines, 
can be operated in parallel if connected as shown in 
Fig. 369. Equalizer connections should be securely 
made and the equalizer itself should have as low 
resistance as possible. 



Fig. 368. — Two Series-Wound Generators in Multiple 
with Equalizing Connection. 

If machine No. 1 is gradually becoming over¬ 
loaded, we must start No. 2 and throw the two ma¬ 
chines together. To do this, bring No. 2 up to speed 
and adjust its voltage until the same as No. 1, then 
close the equalizer switch, and finally the double 
pole main generator switch. Then adjust the volt¬ 
age by means of the shunt field rheostat until the 
incoming machine takes its proper share of the load. 

To shut down either machine, first reduce its 
voltage until it is carrying practically none of the 
load, then open first its main switch, and afterwards 


344 











Central Stations 


the equalizer switch. Then its engine may be shut 
down. 

Alternators cannot be easily or satisfactorily 
operated in series unless their shafts are rigidly con¬ 
nected together to insure their keeping in phase. 
However, there is very little, if any need for series 
operation of alternators, as transformers can be used 
to increase the potential to any desired figure. 

For multiple operation, they must be synchro¬ 
nized before being thrown together. Lamps can be 
used for this purpose, as indicated in the diagram, 



Fig. 369. — Two Compound Generators in Multiple with 
Equalizer. 

Fig. 370, though the best practice favors the use of a 
synchroscope or synchronism indicator, even though 
the cost is greater. In starting and connecting 
machine No. 2, it must not only be brought to exact¬ 
ly the same speed as that of No. 1, but into such 
relation that its voltage wave will be in phase with 
that of the first alternator. When machines are in 
phase there is no difference in voltage between 
similar points such as A and C, but full voltage be- 
A and B. Consequently if the secondaiy 
voltage of each transformer is 110, the two lamps 


345 














Electricity and Electrical Apparatus 


will receive 220 volts and will be at full brilliancy. 
If the leads to B and C are interchanged, the lamps 
will be dark at synchronism. As 2300 is a common 
voltage for alternators, transformers are shown in 
the diagram, the lamps being connected in the sec¬ 
ondary circuits. 

As lamps do not indicate definitely the exact 
point of synchronism, machines might be thrown 
together that are slightly out of phase, causing heavy 
cross currents between the machines, and tripping 


jBUSBARSJ 

IL bwiTCH 



Fig. 370. — Two Alternators in Multiple. 


of oil switches. To avoid this, synchronizers are 
used. Fig. 371 illustrates the general external ap¬ 
pearance of a synchronism indicator made by the 
General Electric Company. 

With reference to Fig. 372 it will be seen that 
this instrument is nothing more than a small deli¬ 
cately arranged motor. The laminated field AA is 
energized by the coils BB which are connected to 
the bus bars on the switchboard through a potential 
transformer. The armature winding consists of 
two coils C and D, connected in series with and at 


346 











Central Stations 




right angles to each other. The outer ends of these 
coils are connected to collector rings 1 and 3 which 
are on the end of the armature shaft, while collector 
ring 2 is joined to the junction of the two coils at Y. 

The primary of the right-hand potential trans¬ 
former is connected to the machine which is to be 
synchronized and thrown in on the bus bars so as 
to operate in multiple with the machines already 
running. The secondary of this transformer is 
connected as shown to the armature and the effect 



Fig. 371. — Synchronism Indicator. 


of the resistance R and the reactance X is such that 
the current in coils C and D are practically 90° apart 
in phase. 

The current in the coils BB and the consequent 
field magnetism is 270° behind the E.M.F. of the 
bus bars — 180° lag on account of the potential 
transformer and 90° lag on account of the induct¬ 
ance of BB. The current in D is 180° behind the 
E.M.F. of the incoming machine and the current in 
C is 270° behind the incoming machine. 


347 


y 


Electricity and Electrical Apparatus 


If the incoming machine is in synchronism with 
the machines on the bus bars the voltages of the two 
potential transformers are in phase. The current 
in C and the field magnetism are in phase and a 
torque would be exerted between them so as to cause 
C to assume a vertical position. As the current in 
D and the magnetism of the field are 90° apart there 
is no reaction or turning effort between them. Thus 
at synchronism the pointer is stationary and up¬ 
ward. 



Fig. 372. — Diagram of Connections for Synchronism 
Indicator. 


If the incoming machine is 90° out of phase 
there will be no torque between coil C and the field, 
but the current in D will now be in phase with the 
field flux and the ensuing torque will cause the coil 
D to assume a vertical position. The pointer has 
revolved 90° either to the right or left, depending 
on whether the incoming machine is above or below 
synchronous speed. If the machines are not out of 
phase quite 90° the pointer assumes a position to 
correspond. And as the phase relation of the in- 


348 





































Central Stations 


coming machine and bus bars constantly changes, 
the pointer revolves. Thus by glancing at the in¬ 
strument when in operation it may be ascertained 
whether the machine that is being synchronized is 
running too fast or slow, also exactly when it is in 
synchronism. 

For convenience in operation, the controlling 
devices, switches, circuit breakers, instruments, 
field rheostats, etc., are assembled or mounted to¬ 
gether on a switchboard of marble, slate or other 
insulating material. 

Switchboards are often divided into sections 
or panels. A generator panel should have a main 
switch for connecting to the bus bars, a circuit break¬ 
er or fuses or both in series with the main switch to 
protect the machine from overloads or short circuit, 
a field rheostat to control the voltage, and a volt¬ 
meter and ammeter. In some cases a field switch 
is also mounted on the generator panel, with aux¬ 
iliary contacts and resistance to take the heavy 
inductive discharge, so that the machine can be 
instantly “ killed* ; in an emergency. 

On alternating-current panels, ammeters are 
usually connected in each phase; thus for a three- 
phase machine, three ammeters should be used. On 
A.C. machines of over 750 volts, it is customary to 
connect ammeters and voltmeters in the secondary 
circuits of current and potential transformers. The 
instruments are thus insulated from the high voltage, 
eliminating danger to the switchboard attendants. 

Feeder panels generally have a fused switch for 
each feeder circuit, and sometimes a circuit breaker 
and an ammeter. 

The exciter panel in an A.C. plant is usually 
equipped with a main two-pole fused knife switch, a 
field rheostat, voltmeter and ammeter. 

It is usual to mount a pilot lamp on each panel, 
so placed and shaded that it will illuminate without 
obstructing the view of the instrument scales. 


349 


Electricity and Electrical Apparatus 

Fig. 373 illustrates a small slate panel, mounted 
on supports of 1" gas pipe. Mounted on a swinging- 
bracket, the voltmeter position can be shifted, en¬ 
abling it to be read from any part of the room. 




Fig. 373. — Small Switchboard Panel. 

Fig. 374 represents a switchboard such as 
would be used in the lighting plant of a large hotel, 
apartment house or office building. This board is 
designed to control one small and two large gener¬ 
ators, and to supply forty-two feeder circuits of 
varying capacities. Each generator is controlled by 
a double-pole I-T-E circuit breaker, and three 
single-pole knife switches. Each is also equipped 
with an ammeter, a field rheostat, and a pilot lamp 
which burns only when the machine is running. 
Note that the two larger machines are idle, their 


350 














































Electricity and Electrical Apparatus 

switches being open, the smaller machine alone 
showing a reading on its ammeter. 

By turning back and consulting the load curve 
given in Figure 365, it will be readily appreciated 
that during the time from midnight to 3 or 4 P.M., 
the output of the plant is small, being about 175 
kilowatts, and yet the expense for labor, etc., is 
practically the same for this small output as it would 
be for twice the amount, or 350 K.W. If the plant 
is offering 24-hour service, it can be readily seen why 
day-load motor customers are given cheaper rates. 
In many small lighting plants this is a perplexing 



Fig. 375. — Storage Battery of Ten Cells, Electric 
Storage Battery Co. 

question. The increase in business due to con¬ 
tinuous operation does not seem to compensate for 
the increased expenses, yet if the plant is operated 
only from 4 or 5 P.M. to midnight, there may be a 
number of consumers who would become dissatisfied, 
to say nothing of making it very hard to get new 
customers. Here it is that a storage battery is 
oftentimes used to advantage. 

As indicated by its name, a Storage Battery or 
Accumulator is a device in which electrical energy 
can be stored or accumulated (as chemical energy), 
and used as desired. 


352 








Central Stations 


Essentially, in its common form, it consists of 
two plates, immersed in dilute sulphuric acid, con¬ 
tained in a jar of acid-proof insulating material, such 
as glass. On portable batteries where weight is 
objectionable, hard-rubber jars are sometimes used. 
In large central stations, the cells are often made of 
planks and lined with lead. 

The positive plates are of lead, the negative of 
lead sulphate. As current is forced through cell, 



Fig. 376. — Large Storage Battery, in Chicago Edison 
Company’s Plant. Manufactured by the Electric 
Storage Battery Co. 


a counter E.M.F. is developed, similar, in a way, 
to the counter E.M.F. of a motor. This gradually 
increases, and will eventually reach approximately 
2.5 volts per cell. After this point is reached, the 
positive plate has a dark brown appearance, due to 
the coating of lead peroxide. It is useless to attempt 
to charge the cell further. 

If disconnected at this time and connected to a 
circuit, it will supply or give out about 80% of the 


353 












Electricity and Electrical Apparatus 

energy furnished it in charging. In other words the 
efficiency of a storage battery is about 80%. 



Fig. 377. — Load Curve, Storage Battery Used to Carry 
Day Load. 

























i/yv) 

























/LW 

i 50 




















Y7, 
























M 





























— 

bOO^ 

























!-§ 



















V/ 






400^ 



















YA 



, 














. ■ I . \ 






































zoo v 

















































0 T 


'Z 2. 


4 - <b © 

A.M. 


/z 4 

/VaorJ ~\~ 


4 - <S <3 

^R.rr. >- 




7?%,^ 


Fig. 378. — Load Curve, Storage Battery Used to Help 
Generators Carry Peak Load. 

In preparing the dilute acid solution, or “elec¬ 
trolyte/ ’ acid should be poured into water, not water 
into the acid. 

The storage battery consists of a number of 
cells in series — enough to give the required voltage. 


354 






































































































Central Stations 


As shown by the curve, Fig. 377, they can be charged 
in the evening, and if of the proper size, can be used 
to supply the demand for current from midnight to 
2 P.M. while the plant is shut down. 

A still more important use of storage batteries 
is to “float” on the line, being charged in the day¬ 
time. At peak load, in the evening, it helps out the 
other equipment, enabling a plant to carry a much 
larger peak load than otherwise possible. 

Due to the large overload current capacity of 
storage batteries, they are a valuable asset in case 
of breakdown, having in some cases on record 
carried the entire load of the plant for a short 
interval while temporary repairs were being made. 

Portable storage batteries are used for automo¬ 
biles and similar purposes. To do away with the 
excessive weights of lead plates and the destructive 
action and objectionable fumes of sulphuric acid, 
Mr. Thomas A. Edison has developed a storage bat¬ 
tery in which the electrolyte is a caustic potash 
solution, and the plates are of iron and nickel com¬ 
position. 


355 


APPENDIX I 


The following is an abstract summarizing the resolutions 
defining the fundamental electrical units, adopted by the In¬ 
ternational Congress of Electricians, in Chicago, 1893. 

“As a unit of resistance, the international ohm, rep¬ 
resented by the resistance offered to an unvarying electric 
current by a column of mercury at the temperature of melting 
ice, 14.452 grams in mass, of constant cross-sectional area, 
and of the length of 106.3 centimeters. 

As a unit of current, the international ampere, repre¬ 
sented sufficiently well for practical use by the unvarying cur¬ 
rent which, when passed through a solution of nitrate of silver 
in water and in accordance with accompanying specifications,* 
deposits silver at the rate of 0.001118 of a gram per second. 

“As a unit of electromotive-force, the international 
volt, which is the electromotive-force that, steadily applied 
to a conductor whose resistance is one international ohm, will 
produce a current of one international ampere, and which is 
represented sufficiently well for practical use by of the 
electromotive-force between the poles or electrodes of the 
voltaic cell known as Clark’s cell, at a temperature of 15° C.” 


* In the following specification, the term silver voltameter means 
the arrangement of apparatus by means of which an electric current is 
passed through a solution of nitrate of silver in water. The silver volta¬ 
meter measures the total electrical quantity which has passed during the 
time of the experiment, and by noting this time, if the current has been 
kept constant, the current itself can be deduced. 

In employing the silver voltameter to measure currents of about 
one ampere, the following arrangements should be adopted: 

The cathode on which the silver is to be deposited should take the 
form of a platinum bowl, not less than 10 centimeters in diameter and 
from 4 to 5 centimeters in depth. 

The anode should be a plate of pure silver some 30 square centimeters 
in area and 2 or 3 millimeters in thickness. 

This is supported horizontally in the liquid near the top of the solu¬ 
tion by a platinum wire passed through holes in the plate at opposite 
corners. To prevent the disintegrated silver which is formed on the 
anode from falling on to the cathode, the anode should be wrapped round 
with pure filter paper, secured at the back with sealing wax. 

The liquid should consist of a neutral solution of pure silver nitrate, 
containing about 15 parts by weight of the nitrate to 85 parts of water. 

The resistance of the voltameter changes somewhat as the current 
passes. To prevent these changes having too great an effect on the cur¬ 
rent, some resistance besides that of the voltameter should be inserted in 
the circuit. The total metallic resistance of the circuit should not be 
less than 10 ohms. 


356 



APPENDIX II 


English and Metric Measures. 

In the English system of measurements, 

1 mile = 320 rods = 1760 yards = 5280 feet = 63,360 inches. 
1 rod = 5% yards = 16| feet = 198 inches. 

1 yard = 3 feet = 36 inches. 

1 foot = 12 inches. 

In the metric system, 

1 kilometer = 1,000 meters = 100,000 centimeters = 1,000, 
000 millimeters. 

1 meter = 100 centimeters = 1,000 millimeters. 

1 centimeter = 10 millimeters. 

The following table, showing values of the metric in 
English units, will be of assistance in changing from one sys¬ 
tem to the other. 


One 

Milli¬ 

meter 

Equals 

One 

Centi¬ 

meter 

Equals 

One 

Meter 

Equals 

One 

Kilo¬ 

meter 

Equals 


.001 

.039371 

.003281 

.001094 

.01 

.393708 

.032809 

.010936 

1 . 

39.37079 

3.280899 

1.093633 

.000621 

1,000 

3,280.899 

1,093.633 

.621382 

Meters 

Inches 

Feet 

Yards 

Miles 


1 inch = 2.539954 
1 foot = 30.47945 
1 yard = .9143835 

1 rod = 5.029109 
1 mile = 1.6093149 


or approximately 

M U 

a u 

u u 

(( C( 


2.54 centimeters 
30.48 centimeters. 
.9144 meters. 
5.029 meters. 
1.609 kilometers. 


English and Metric Weights. 

1 gram = the weight of one cubic centimeter of pure dis¬ 
tilled water at its temperature of maximum 
density. 

1 kilogram = 1,000 grams = 2.2046 pounds. 


357 





























































































* 












